I DM'd quite a bit back in the 70s.  After about 83'ish I haven't played again.  However, I am newly retired and have noticed these new sites like Roll20 and D&D Beyond where one can game with people all over the world. 

Back in the 70s I was a popular DM known for rich, vibrant worlds.  We did D&D plus many other types of games.  Over the past year I have tried 3 campaigns and they both have failed to some extent...but I consider them failures.

- The first was just because I took whoever signed up.  I learned my lesson to interview players beforehand and to be a little ruthless in not allowing some people in.  The group fell apart quickly because bad players/trolls were allowed in.

- The second failed because the campaign was too tough.  I knew it might be from the original idea that I did in the 70s and so I greatly eased it up on difficulty.  The group just couldn't seem to overcome even the simplest, easiest obstacles.  I coddled them along for a bit but when they blundered into an extremely obvious bad situation, I had to kill them and then dissolved the group.

- The third failed because it was too railroady.  I was also told it had too much combat.  People today seem to not like combat as much as in the past...The campaign will still work, I think, but needs work to get rid of the more railroady aspects.

So...I have a new campaign.  Not railroady, some combat but toned down.  Open world-ish but with a story/machinations behind it.  Hopefully not too hard in thinking but can be scaled up/down in difficulty depending on the group....

However, I have another problem creeping up.  One I would like some advice on if you have it...

The new problem is that my campaign starts out with the PC's on a mission.  It literally starts out in the character generation phase with a job interview.  The campaign is set in the future with the PCs sent in to investigate a sector that has an illegal activity/drup problem.  So it is a undercover cop/drug/criminal campaign.  In order to be 'hired' (and admitted into the campaign) one must make a PC that fits with who would be hired.  The problem?  3 people have failed the interview...even though they knew what the campaign was about and I attempted to guide them in the character creation process.

- One insisted that he wanted to pilot 'Mechs' in combat.  He was disappointed that he wouldn't start with a mech (though he could make money and make his own for one later).  It was obvious he didn't want to play in THIS campaign, but a mech-based fighting one...

- The second insisted on some convoluted back story that had nothing to do with what they would be doing.  Without going into it, there is NO WAY they would be hired into this mission.  Tried to get her to try something else...very resistant.

- The third wanted to be some sort of extremely powerful Jedi thing or something and was disappointed he would start out not as superhuman.

In short, they all wanted to play but what THEY wanted to play.  Why they applied to my game...I just don't understand.  I think I was perfectly clear what the campaign was going to start as...  Is this a problem others have?  If so, what is your advice for dealing with it?  Maybe I just had some bad luck?

The expectations of the game have certainly changed.  Players in general seem less inclined to simulation and tactics, I agree.  That doesn't bother me, personally.  They also have an expectation of being partners in telling the story, if not in building the world.  Your account of the second group seems to me to imply a slightly antagonistic tone, is that fair?  I mean when players make bad decisions, they wipe, I've done it myself; but then the group has a funny story about the TPK, it doesn't have to be dissolved.

Have you looked at the way other DMs run games?  It seems like every other person is streaming their games on Twitch.  I've gotten some useful insights from them since I started getting back into 5e.

I don't know what you advertised the campaign to be, but it seems you're going for a very William Gibson meets The Wire vibe that has no time for whimsy or playfulness.  You can see how that might be a little heavy for generations raised on a more colorful, poppy, kind of sci-fi.  Why anyone who wanted to pilot mechs would sign up for Donnie Brasco, I don't know; but my first thoughts are that they just heard 'future' and ran with it.  

What's my advice for dealing with it?  First, I'd maybe just engage as a player for a bit and try to get a better feel for where people are at.  Make some friends, collect some Discord numbers and put together your own group from there.  Second, I'd look again at how you're advertising the stories you want to tell.  Third, you just have to fish with the bait the fish want.  Don't let someone be a first level Jedi Master, but once they hit fifth level sorcerer, detect thoughts and enhance ability are both very Jedi like powers that would be huge in an undercover cop campaign.  Be sure to tell people that.  A 200' tall battle mech would be way outside your campaign's scope, but your guy sitting in front of a bank of monitors while piloting a getaway car or a surveillance drone or some other "pet" might be a good compromise.  Stuff like that.

These seem like people who aren't really interested in playing a story. They want everything to be about their character (besides the second one, who might've just really liked the backstory). You should specify that there will be no homebrew options besides the ones you provide (if any). Most experienced players don't do that type of thing.

could you go more into detail about the 2nd players backstory and why it might've been a problem

One thing that sometimes people 'forget' is that the DM is a player at the table, too.

If you as the DM want to play this particular campaign, stick to your guns, keep interviewing, and most importantly, reject those who do not fit in.

Often, players assemble with their own character concept, and leave it up to the DM to create a campaign that brings all the different characters and their concepts together seamlessly.

Some DMs like to accommodate just that, see it as their challenge, and derive enjoyment from it.

However, that's not the only way to run a game, nor are you required to make yourself fit into that box.

If you want to play your campaign, I suggest you run a series of unrelated one-shots in your world. Maybe even only allow pre-made characters for those sessions, and find players who enjoy your style and world. If people from the one-shots don't fit, don't invite them to the campaign. If you have a good player in the group, extend an invitation. 

I started playing right around when you stopped, back in ‘83. Yes, the game has changed a lot. People (generally) expect their characters to live all the way from beginning to end of the campaign nowadays, for example. Also, there’s this idea of player agency, where people are assumed to have a bit more freedom in creating their characters. For example in olden times, we’d roll ability scores and see what we could make. Now you decide what you want to make, and fit the scores to the concept. (I’m not trying to re-start the argument about which is better, simply observing that it represents a sea change in character generation). That sounds like something you’re running up against; people expect much more latitude in what they can make. Certainly DMs can place limits on this, but that’s something to work through with the players.

To get to your more specific issues. One thing that’s really come along they call a session 0. You and your players talk through what kind of game you all want to play. So that campaign where you had too much combat — that might be more particular than universal. Some groups really want lots of combat, and don’t care how thin the story is that connects the fights. Others want lots of role playing and story, punctuated by the occasional fight. Session 0 is where you pin that all down before you really start. And I’m just using that as an example of what you bring up but the combat vs. role play continuum is one of the biggest things to address. Other things could be if you are allowing evil characters, or if people have any personal issues that really upset them, which they do not want to see in game.

With the characters for this new campaign, again, could maybe be fixed by session 0. But my bigger suggestion would be to be more specific with what you want to suggest for characters. As in: Your characters should be one of the following classes/subclasses, because it seems like you have some very specific things in mind.

I have been going down the thought path that some of you have brought up...that being a very short campaign/one shot in the world.  I thought I was doing this going for a 9-12 session couple month sort of thing but I think I need to go much further.  I am going to seriously consider making up a very short/one shot scenario...this way I can learn and also using it as a 'screening' process to find people that like it.

The personal issue I think I am going to have to deal with is I am a 'planning' DM.  I can roll with it/ad lib as well as most, I think, but I don't really like it.  I like to 'ad lib' within a good framework already in place.  I like to have a world that has substance behind it.  I know I get disinterested if I am a player in a campaign and I get a strong feeling the DM is ad libbing most of the time.

An example of this is using the show "The Expanse'...which bears a significant resemblance to one of my more popular campaigns in the 70s.  A party doing something I didn't expect I can deal with very well, especially if it is a good/creative idea.  I think I can roll with it an adapt very well...however, if the crew of the Rocinte (spelling) decided screw this proto-molecule thing...let's go check out Pluto for  no reason....no...just no :)

Another thing to consider is using a different game if you’re doing sci fi. Re-skinning the D&D rule set can get you there, yes. But starting out with a game designed for sci fi from the beginning will probably work much better and have less need for homebrewing.

I think this is a "them" problem and not a "you" problem. Weird, entitled randos are one of the main reasons I don't play online, but I can't speak to exactly how common they are. Do you have any friends who might be interested in a game night every week or so? They might be more casual than the people you'd find online, but that's not a bad thing at all. People who come to the table without expectations are often the best players, and if you already have fun hanging out with them, so much the better!

Quote from NaivaraArnuanna >>

People who come to the table without expectations are often the best players, and if you already have fun hanging out with them, so much the better!

 I think this is one of the best points mentioned in this thread so far.  If both DM and players have a lot of goals or expectations, someone is going to be disapointed.

Before COVID, I'd never used online gaming platforms.  In the last year, I've run 3 different games of D&D totaling almost 50 sessions. Some players are people I've gamed with before in-person; others are friends of friends who I've never gamed with. One campaign is using the Adventurer's League Storm Kings Thunder modules which dmsgulid offered for free last spring; the other two are home-made adventures based in Forgotten Realms.

Observations based on my experiences:
1)  As Naivara said above, HAVE FUN!

2) Everyone should get something they want but some players are too demanding to "play nice with others".   Out of 10 players, I've only kicked one out after he "blew-up" multiple times.

3) Maybe try to keep things simple (at least at first).  I started both my homebrews off with just notes on a few simple adventure hooks rather than a "large-scale" campagin idea.  I then built the following sessions and campaign off of the choices the players and their characters made.  I try to have an outline of where the campaign is going, but only actually write details about 2 sessions in advance. That way when the players take a left turn like killing off a major NPC or not taking the "bait" on a hook, I don't have to rewrite a ton of material.

4) Times have indeed changed.  I started the hobby with AD&D and Moldavy BASIC, if your 3rd level adventuring party found a dragon in the dungeon and was eaten; well, that's how the dice roll.  Many modern players and DMs would be up in arms if such a thing happened to their adventuring party.  What hasn't changed is they all want to enjoy themselves (see #1 above). Know who your players are and what their motivation is. The old stereotypes such "power-gamer", "role-player" and "looney" still run true, some players want to kill everything, others only have a character-related goal, some just want to have a "good time" which could mean anything from killing a goblin by dropping a piano on it to causing a total party-wipe.

Have you had a session 0? If it then I suggest you sit down with your players and have one, the whole point of the session is for you to understand what your players want from a game so you can tailor it accordingly. There is loads of info about a session zero online so I won’t go into details here have a google and a read but over 20 years I have pretty much always started every campaign of every system I have run with some kind of session 0 and found it really helps. 

I'd say the interviews are in fact personally catered session 0s for each player, and it sounds like the world was outlined ahead of the interviews.  There is something to be said for creating a world that speaks to player's interest ... I do that myself.  However, there's also a lot to be said for players being introduced to a world of the DMs design and enjoying the exercise of building a character who fits into that milieu.  I've also done that.

It sounds like however the OP presented the world, for whatever reason the players just didn't get it.  That's possible because they all sound like they're playing power fantasies rather than edgier grit characters that an undercover cop game would entail.  Players who have no real world exposure to paramilitary or military bureaucratic structures just aren't going to get it these days without some assist.  I'd provide some media aids to help get the tone and scale/scope of the game across.  Off the cuff I'd use the training academy scenes in The Departed, Gaff picking up Deckerd and meeting with the brass in Bladerunner, and the opening scene between McNulty and a witness over the corpse of Snotboggie in The Wire, to see how Jedi and Mechajocks don't really fit into scale and scope of the game.  As for the convoluted background story, just explain that agencies like the one characters work for work hard to do a background investigation to determine suitability, and that sort of background wouldn't be suitable for "the job."

There are players who would want to play in that sort of world, but frankly I'm not sure modern D&D player sensibilities are the best fit.  That's a real broad brush, but I'd say in terms of promoting your game, folks who play Cyberpunk, Delta Green, or Call of Cthulhu I think might get that it's more about finding a fit in an established world often through desperate compromise.

I mean given the scenario's you'd outline I'd say "This game is a gritty noir about infiltrating criminal enterprises, a mechajock is going to have nothing to do but sit around and wait for the final bust, if that level of force is authorized.  A Jedi clearly would not have comfort with the moral ambiguity this sort of undercover work entails (thought there is that cool Clone Wars arc where Obi Wan did in fact infiltrate an association of bounty hunters deep cover style), and for the backstory one, yeah, there's no way a character with that many complications in life would make it through a background investigation.  If they can't adapt to the game you want to play (and do the labor of the DM), you just need to find the right players, again, who are probably playing different rules systems.

TL;DR I have no idea what sort of DM you may be and the comments may come across as harsh. It could well be that everything has been misinterpreted. However, I'd suggest taking a close look at your DMing style to see if that could be a factor in the issues you have encountered rather than it being a fundamental issue with the players.

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1) Are you playing D&D 5e? The start you have proposed for the campaign doesn't sound like it but it could take an unusual twist. Is it a science fiction game?

2) You didn't include the description of the campaign or the game system that was available to the players in order to create characters. Does that game system include mechs, jedi powers, and other features? If it does then it might be reasonable for folks to expect to create characters with such features. You are running some sort of police drug/criminal future game but that doesn't necessarily mean that every player has to choose to be a detective type with no interesting aspects to their background, previous employment or other aspects that may or may not be a part of whatever game system was in use.

Anecdote: I once played in a very short lived campaign in which the DM had in mind some sort of adventure based in a middle ages fantasy setting and they were going for realistic. Everyone in the party was forced to be a man at arms. We spent the days either guarding at the keep or out on patrol. Every character was similar to every other one except one character who had a bit of magical ability (low magic campaign). It was one of the most boring experiences I have ever had playing a role playing game and it folded after something like 4-6 sessions - at least I didn't play any more. This was a case of a DM having something that sounded brilliant in their own head and wanted the players to be minor actors in the story they wanted to write. That isn't D&D or any role playing game in my opinion.

3) I have been playing 5e for about 4 years (DMing for a bout a year and a half) after a few decades with only the occasional one shot. I first played and DMed in the late 70s. I am playing with folks I knew in high school and well as groups I met at a local game store, and other acquaintances. Ages range from teens to 60's. The game systems have changed, the players, in my experience, not so much. Some players who are coming from a video game background are a bit less invested in "realism" and just want to have fun with a role playing game. Personally, I generally prefer 5e to the preceding versions of the game. Some players love role playing, some love tactical combat, some like min/maxing, others could care less - that was the same in the 70's as it is today.

If you want to run a game for folks with certain preferred playstyles then you need to see if you can recruit folks like that.

4) In your descriptions of the issues you have encountered ... most of the problems seemed to be with the DMing rather than the players.

- "The first was just because I took whoever signed up."

This can be a problem because you get a bunch of folks with disparate play styles. However, most folks who sign up to play a game sign up because they actually want to play. There may be a few "trolls" out there who just want to mess up campaigns but it becomes pretty obvious to both the DM and the other players when this is happening and it is easy enough to exclude them from future sessions.

- "The second failed because the campaign was too tough.  I knew it might be from the original idea that I did in the 70s and so I greatly eased it up on difficulty.  The group just couldn't seem to overcome even the simplest, easiest obstacles.  I coddled them along for a bit but when they blundered into an extremely obvious bad situation, I had to kill them and then dissolved the group."

Wow. Folks decide to invest their time playing a campaign you built so when there are design flaws because it it too difficult "The group just couldn't seem to overcome even the simplest, easiest obstacles",  you blame the players "I coddled them along" ... and then kill them and end the campaign "I had to kill them and then dissolved the group". These folks took hours out of their lives to try playing your game and yet you clearly didn't think much of them and kill them off.

Having run many groups over the years, puzzles that the DM thinks are "easy" or "obvious" are rarely so to the players. The DM has all the pieces, the players don't and the so called easy problems aren't. Some DMs realize this and others blame the players.

- "The third failed because it was too railroady.  I was also told it had too much combat."

Being railroady is generally not popular but some railroading is generally tolerated unless the situation is one where the DM literally gives the players little or no choice about what they can do. Railroady kill campaigns only when the design is totally flawed and the players are literally left with the feeling that none of their character choices really matter. As for combat, that is again a playstyle issue. I know some folks who love combat, others who are less enthusiastic, but almost every D&D session will have some form of combat so unless you were running a game that just went from fight to fight to fight (and was a fighting railroad) then usually it isn't much of an issue.

5) On the topic of your new game ...

Three players come up with character concepts and present them to you.

- a character that uses mecha - what's wrong with a former mecha pilot as a background? Seems like it could work in law enforcement. Did you let the player know that there wouldn't be mecha in the campaign? Was this included in whatever information you gave the people to create characters?

- a character with some sort of mental powers - where did the player get the idea of mind powers? Is it supported by the game system you are using? Did you let the players know that only certain types of characters would be allowed?

- a character with an involved back story - how did the back story disqualify the character for a detective job? Or was it the DM who just didn't like the idea?

You have a job posting for investigators - WHY would some total newbies be hired to investigate some sort of criminal/drug thing without training, right off the street? Did you require that every character be trained detectives, ex-military, former police or private security? Did you tell the people who expressed an interest that you were only interested in players that would play specific types of characters and not other ones?

Finally, it can be difficult coming up with a group of players these days who can get along and play together with compatible play styles and interests. There are far more people playing now and many have been brought into the hobby through web shows like Critical Role or Dimension 20 or others. This results in a wide range of expectations of what play is like and the pandemic has not made it any easier. There are lots of folks looking to try out games and some looking for new games to play. Finding a group that "clicks", gets along, has the time and motivation to show up regularly is hard. All of this requires a lot of flexibility on the part of the DM. They are telling a cooperative story based on the content they create and the characters run by the players. Some players will need hand holding on role playing and game mechanics. Others will need help in speaking up and others the DM will have to help be quieter so that others get a chance to play. This means that the content the DM designs may need to be modified on the fly, or may need to change as the game evolves, plot lines may shift as the players make different choices. The DM adjudicates the interactions of the players with the world they have created and the result is a story that is jointly written by the DM and the players.

However, some DMs decide on the story they will tell. They decide on all the pieces and what will happen. Sometimes the players will have the appearance of free will and other times they may feel railroaded. In this case, players may decide to goof off because they aren't having fun or because they realize their choices don't matter, everyone may die because there is only one way for the campaign to run and that is the way the DM decides it must be, the players may feel overwhelmed because the game feels like a railroad or the players may come up with character concepts that don't fit because the DM has already decided what sort of characters are needed to create the story that is going to happen.

I'm going to be honest: this sounds an awful lot like you've got specific ideas for how you want to run your games and you're insisting that players follow everything you want them to do instead of working with them to see what they're interested in doing.

I'm sympathetic to what 6th and David42 are saying, but at the same time I'm understanding you're pre-invested in the game world, and by default more invested in it than you are your players.  I don't think this is necessarily a bad thing, and also understandable since your players are strangers.  Honestly, I think it might make more sense to design a suite of pregens, a big enough array that the players have options to choose from.  Give them a little wiggle room to customize stories and maybe some mechanical features and run your game from the outset like that.  If they like playing "your game" as the game develops they might want to bring in new characters to help expand the world at which point it become "your game" more collectively.

I've seen streamers try out new games all the time with pre-gens, it's often a great way to intro the game and gets players a better sense of what they're signing up for before doing the work of making characters on their own.

It is perfectly fine to for a DM or GM to set parameters to their world, especially when it comes to genre tropes (for example new mecha, no Jedi, backstories no more than two sentences because that's not the point of the particular game, etc.).  However, as you're learning the more set your parameters are, the harder it will be to find players who want to go on that particular ride.

I'm not sure this has anything to do with "times have changed" besides the fact that these days it's much easier to play with random people you don't know, which has aways been a hit-and-miss experience. I assume that's what these games are. We don't all have the luxury of playing with people we know, but if you have the opportunity I highly recommend it over a pick-up game.

DMs are a rare commodity. A lot of people (of widely varying maturity and expectations) are going to jump on any open game they can get. I would suggest that you put a lot of info into whatever method you are using to recruit players. Be upfront and specific about what you're looking for and be clear that a good fit is required for entry.

One other impression I get from your second group is that there was probably a communication issue there. They might not have been able to overcome simple obstacles due to the fact that they weren't seeing them as obstacles or weren't sure what they were supposed to do.

Are your old gaming buddies available to play?, you could have a lot of fun playing with them, campaign style is known, and you‘re among friends.                                              

It sounds like you need to be stricter with what players you're accepting into your campaigns or you're not giving a good enough description of the campaign. I'd recommend clearly outlining in the looking for players post that you plan on running a combat-heavy game. Give your description of the game a once-over like you were seeing it for the first time as a player and try to figure out if you could grasp what was being said from it. Also, list any restrictions or specifications you may have for the game as close to the top of the post as possible.

As for the players, the best advice I can give is, ask every player what they expect from the game and what character ideas they have. If a player can't fill out those two things when asked to, they're not considered. If a player's expectations are vastly different from your own they're out of the running. If their character idea doesn't fit but they otherwise seem like they could be a good player I'd flat out tell them that their character idea doesn't fit and if they can make a character that does fit they'll be considered. It might sound a bit harsh on the players but it's the reality of playing online if you want to find a good group for your campaign. In addition, don't be afraid of losing a few players within the first few sessions, keep a few people who applied and seemed good as backups just in case. 

Ah, the old, old days... I have been around for much of this period, although largely as a player rater than a DM.  

I think the key -always-- is to find a compatible group, one with similar ideas on what aspects of the game they most enjoy and what kind of story you will be telling together.  I am very lucky in that my core group has been together now for years and we share many of the same tastes.  The disruptive new player from the past campaign who wanted to be all powerful and had ignored the "no preying on party members.  We will have enough to contend with in a horror setting", did not return.  (I'm not sure if that was due to the paladin leaving his character behind in the Abyss since 'she belonged there' or because the new DM  insisted he would have to roll stats at session 0 using the same method as everyone else.) 

Our other new player is still learning rules, ("I though it was all over once you started to drown." / No, you still have a few rounds..")  which is definitely not a problem, and he has been a great addition to the group.  

I think your idea of interviews is a good one.  Just stick with it and try not to be discouraged.  It may take some time. [My second ever campaign disintegrated after the "paladin" murdered a sergeant of the guard who tried to question the party.  His character was jailed and awaiting trial when the player left the group taking his girlfriend with him and telling me "You won't have any game at all without players" // In fact it was two to three weeks later the rest of us resumed playing with a couple of graduate students.  That group was together until they both finished school and moved out of state.]

So, while the game rules have certainly evolved, I don't think the fundament search for the right group is any different now.  It can be hard, frustrating and time consuming to find your fit,  but keep trying.  It will all be worth it in the end.