I've included a mystery in my Strixhaven game that I've ensured has a way to solve it if the players approach the problem correctly and approach it from all angles.  Basically the school newspaper printing press is possessed by a vindictive spirit of the former editor and is inserting a series of vicious attack articles against students that are the children of the survivors of the accident that killed him. 

It's early in the players' investigation, and currently their plan is to catch the perpetrator in the act by staking out the newspaper office the night before the new edition goes out. Since this is a supernatural crime, the articles really just appear in the paper because of ghost stuff, so there's nothing for them to really see, and I planned it out as sort of a slow-burn kind of mystery so they shouldn't be able to solve it first thing they try anyways-- they need to think about it to solve it. 

That said, I don't want them to feel discouraged when their plan reveals nothing and the ghosts next gossip article appears in the paper regardless. I've dropped a subtle hint before of something supernatural in the Star office, told a player that they "didn't feel alone... like they're being watched" when they were there,  but I don't want to lay on the hints on any stronger than that because I'm not sure I want them to come up with the ghost conclusion before figuring out motive, especially if it's just something I more or less handed them.

Do people have experience running supernatural mysteries that lack physical clues, and advice on keeping players from getting discouraged? 

(For those interested, they way to figure out what's going on is to interview the victims and look for what connects them. Doing so should turn up that they're all legacy students, and their respective parents all went there around the same time, about 25 years ago. If they go through the backlog of of Strixhaven Star copies (which an npc might suggest if they're hitting a wall there)  from around that time, they can learn about an accident that killed a former Star editor that the parents were the only survivors of. The former editor had been sensationalizing events to drum up tension between the colleges of Strixhaven in order to create more dramatic news and increase paper circulation. This campaign backfired, as a quandrix experiment was sabotaged by prismeri rivals, and the experiment killed everyone involved except for the current students parents (the editor was there to report on the failure, having been tipped off about the sabotage in advance) . The ghost blames the survivors for what happened because their ego is just too big to acknowledge it was their own fault, so the ghost is trying to make the children miserable as revenge).

If this former editor is very evil, for lack of a better word, I suggest you have Thieves' Cant hidden in the articles that says things like "[name], I'm coming for you."

This is very cool and sounds like a great story!

I think that after staking out the office, they'll realise that it has to be something coming from inside the office. They'll get there on their own without any further clues, although maybe there could also be an article describing the accident and blaming the survivors (put a red herring character in the article for the PCs to believe may be the culprit, who can then tell them about the former editor's obsession with the printing press). 

I think this touches on something pretty universal. It's not uncommon to want to include some kind of long-term, slow-burn storyline. Everyone loves the movies where a twist changes your perspective and you can look back and see that there were signs pointing to the truth all along.

And you've outlined the difficulty we face pretty well - too much info and they figure it out right away, not enough and they feel they must have failed at something or they just dismiss it as unimportant. What has worked best for me is to be upfront about it, preferably in Session 0 but you can interject any time. Just let the party know that the game may include a mystery or have a story element that requires the synthesis of clues gathered over many sessions. Once they know to shift their brains into long-term perspective when they're looking for clues, I've seen players really engage with this kind of stuff. But many players - especially newer ones - are not going to expect that kind of thing without a heads up.

If the players are logical then they should get there eventually.

1) What copy is supposed to be in the paper? They find this out from the current editor.

2) How does the printing process work? Does the editor just set it to run overnight unattended? How is the text to be printed sent to the printer? Is it like a movable type printing press where each letter needs to be put in place or is the process magical?

3) After verifying the content intended to be printed. Finding that no one they can see is present to change the text being printed and then observing that the text coming out of the printer has the revised text - they should be able to narrow down the issue to the printer itself. Depending on how smart the ghost is though, it may not alter any printed copy while the investigators are present.

4) Is the text in the paper present in every printed copy or does it only get changed magically when someone goes to read it? Are the printed papers enchanted or is the actual printed text on the page change? Depending on how the text is being changed the players could have different clues to it.

However, I think that tracking down the backstory may be hit or miss without sufficient nudging. The players may not think that the particular students are being specially targeted and each of the students themselves may not really be aware of the story - it was 25 years ago - their parents may have mentioned it to them. On the other hand, maybe the families are close, see each other regularly, and formed a strong bond as a result of the event so the students may be aware of the story. However, in this case the students are also likely close to each other or have known each other for years, have their own clique and might not think the attacks have anything to do with their parents. So, the challenge as DM will be connecting the dots without being too obvious while at the same time giving enough for someone to figure it out.

Also, be prepared as soon as a critical number of facts are known for someone to shout out the answer - "It is the ghost of the paper editor that died when everyone's parents survived". If anyone was listening to your comments about it feeling "creepy" they may already be primed to think of ghosts.

Quote from CharlesThePlant >>

I've included a mystery in my Strixhaven game that I've ensured has a way to solve it if the players approach the problem correctly and approach it from all angles.  Basically the school newspaper printing press is possessed by a vindictive spirit of the former editor and is inserting a series of vicious attack articles against students that are the children of the survivors of the accident that killed him. 

It's early in the players' investigation, and currently their plan is to catch the perpetrator in the act by staking out the newspaper office the night before the new edition goes out. Since this is a supernatural crime, the articles really just appear in the paper because of ghost stuff, so there's nothing for them to really see, and I planned it out as sort of a slow-burn kind of mystery so they shouldn't be able to solve it first thing they try anyways-- they need to think about it to solve it. 

That said, I don't want them to feel discouraged when their plan reveals nothing and the ghosts next gossip article appears in the paper regardless. I've dropped a subtle hint before of something supernatural in the Star office, told a player that they "didn't feel alone... like they're being watched" when they were there,  but I don't want to lay on the hints on any stronger than that because I'm not sure I want them to come up with the ghost conclusion before figuring out motive, especially if it's just something I more or less handed them.

Do people have experience running supernatural mysteries that lack physical clues, and advice on keeping players from getting discouraged? 

(For those interested, they way to figure out what's going on is to interview the victims and look for what connects them. Doing so should turn up that they're all legacy students, and their respective parents all went there around the same time, about 25 years ago. If they go through the backlog of of Strixhaven Star copies (which an npc might suggest if they're hitting a wall there)  from around that time, they can learn about an accident that killed a former Star editor that the parents were the only survivors of. The former editor had been sensationalizing events to drum up tension between the colleges of Strixhaven in order to create more dramatic news and increase paper circulation. This campaign backfired, as a quandrix experiment was sabotaged by prismeri rivals, and the experiment killed everyone involved except for the current students parents (the editor was there to report on the failure, having been tipped off about the sabotage in advance) . The ghost blames the survivors for what happened because their ego is just too big to acknowledge it was their own fault, so the ghost is trying to make the children miserable as revenge).

I don't know if it was intentional or not, but the first thing I thought when reading this plotline was Nightmare on Elm Street -- dead villain who feels wronged and comes back to go after the children of those he feels wronged him

The solution in the movie came from one of the parents finally confessing what happened. I'd make sure at least one parent is on campus (as faculty, even) and easily accessible as an exposition source if needed, rather than just having the party piece it together from old articles. If you want an additional twist/complication, make the on-campus parent the original saboteur, so they have a vested interest in not admitting the whole story until the stakes get high enough that they have to choose between their reputation and their kid

I deleted something earlier, but if I'm going to get notifications, I might as well say it:  When they stake out the press - they find a Mysterious Figure skulking around.  The players play it how they play it, but it's some kind of authority figure who was conducting their own examination on the press.  The authority figure gets upset, pulls rank on the students and tells them to go question witnesses.

The next day, the authority figure is dead.  Their neckwear got caught in the printing press.  Their mottled corpse is the  photo on page one above the fold, in color.  Under the fold is your players with a caption:  Losers Fail To Prevent Death!  Killer Roams Free!  Because tumult ensues when the authority figure dies, they don't have any leads to go on OTHER than the leads the authority figure gave them.  Their first instinct my not have been that the printing press was, itself, possessed.  They might think there's a Scooby-Doo situation happening here.  And then you lead them back to the press the way you wanted them to go.

In general mysteries are hard to do in RPGs, because you really don't know what clues your players will pick up on, so if you make it hard enough that they probably won't immediately solve it there's a good chance they'll never solve it, and still a significant chance that they solve it instantly. You can cheat to some degree by putting time/event locks on some clues (they simply can't be found until some time has past or something else has occurred), and conversely, feeding them clues if they're behind (e.g. the classic "try to silence them with thugs who are conveniently carrying clues"), but that starts to be less like an actual investigation and more an obscured railroad.

I think it's fine for them to watch and not see any one come in. That itself is a clue if they are smart enough to realize it. Another clue you could have is have the articles them self use some old slang or rumor that lets people know who ever is doing this is old. As for how the players stop them getting discouraged, you just have to give them a reason to pursue it. Just keep the rumors coming, have them share personal secrets that hurt their reputation or similar. if the quest is impactful they'll interact with it whether its trying to down play the consequences or stopping it once and for all.

So update; our session was on Tuesday and the stakeout went more or less without a hitch. I had another student sneak into the room in the night also trying to figure out who the AoS was for her own selfish blackmail purposes. She, a stereotypical Mean Girl, and the player staking out the office, both confronted each other about being the Author of Shadows and there was some tense back and forth that ended with the mean girl threatening blackmail if the player told anyone she'd been there, and the player pickpocketing her student ID from her pocket.

The rest of the night went without incident, they confirmed the papers hadn't been tampered with, but then later in the day after more campaign shenanigans, a new article appeared back in the paper as if by magic, confirming the tampering is taking place outside of the normal printing process.  One of the artificers cast detect magic on a copy of the paper and I had them roll Arcana in addition to the spell. He rolled high so I told him they senced something "supernatural, not necessarily magical." He actually then said "maybe it's a poltergeist or ghost" without any further prompting from me, but the rest of the party didn't seem convinced and the discussion quickly moved on to other aspects of the investigation. I've had the paper's current editor recommend to the player that's been leading the investigation that they should look into what connects the victims rather than unraveling how it's being done, so they bribed a faculty member (a slightly mad, paranoid Laboratory Faculty Supervisor by the name of Murgaxor, who the students are on decent terms with and they don't know anything about his "extracurricular activities" yet) into getting access to the three victim's class schedules. 

I'm going to sit back for a session or two i think, and let them drive the action and persue the mystery as they want to and in the mean time I'll run a few bits of the book adventure, and see what form their investigation takes. Even if they do come to the conclusion that it's a ghost early, which they haven't really yet, they still need to figure out the backstory to figure out how to draw the ghost out of the press (by re-creating the experiment that killed him), so they've still got plenty of mystery left.