Items such as the longsword, a regular equipment item in the basic rules, simply don't appear in the add items section of the character editor even though they do show up under the items/equipment menu

I'm having this problem too. But it doesn't seem to have anything to do with proficiency.

For example -

I can't find Potion of Fire Breath or Elixer of Health when searching with a Dwarf Wizard, but can with a Earth Genasi Paladin.

I couldn't find Shatterspike with either and ended up just rebuilding the characters assuming it had something to do with the their age (we haven't played in almost a year).  now my paladin has Shatterspike but my Wizard is without his potions and elixer.

I'm really hoping I'm just being an idiot, but as of right now I can't figure it out.

Edit: Nevermind. I am indeed an idiot.  I had disabled content sharing when we quit playing because we started a different campaign.  I'm stupid.

I have been having similar issue 

I have tried adding the sentinels shield to my inventory 

I have content enabled and shared for the campaign and the character is part of the campaign 

All filters are off     

I can't find the shield just fine outside of the characters inventory 

Is your character proficient with shields?

yes he is

but i also tried toggling on and off the proficient button  and trying on a different character in the same results

in Avernus we gain a Charm of Vitality, its not in dndb inventory even though I have the book how to fix this

Quote from Hildahl >>

in Avernus we gain a Charm of Vitality, its not in dndb inventory even though I have the book how to fix this

 Charm of Vitality is not listed in Descent into that I know of. Is it something your DM made up, or did they get it from a non-official book not purchasable on DDB? Either way, you’re gonna have to create it using the homebrewer. If you tell me what it does, I can tell you how to make it.

Quote from IamSposta >>

Quote from Hildahl >>

in Avernus we gain a Charm of Vitality, its not in dndb inventory even though I have the book how to fix this

 Charm of Vitality is not listed in Descent into that I know of. Is it something your DM made up, or did they get it from a non-official book not purchasable on DDB? Either way, you’re gonna have to create it using the homebrewer. If you tell me what it does, I can tell you how to make it.

 Charms aren't items. They fall under the category of Supernatural Gifts 

Other Rewards - Dungeon Master's Guide - Sources - D&D Beyond
https://www.dndbeyond.com/sources/dmg/other-rewards#SupernaturalGifts

like you get from a Chwinga.

Like divine gifts and the special character gifts from Theros, i don't think the system currently has a way to automatically apply the mods directly to your character sheet, (although  they're working on it.) 

Until the proposed changes have been made,  you're going to either have to use homebrew or manually customize your character sheet. The notes page is a good place to explain the customization. 

Edit: just reviewed that charm. Since it's a single use action to trigger, just list it on your character sheet as Potion of Vitality.

I am unable to add items from the Player's Handbook even. I am looking at the "Ammunition, +1" and I cannot find it to add it to the character. So it's rather an issue of what the character sheet supports at the moment and not what is unlocked in order to do inventory management?

Quote from SolFolango >>

I am unable to add items from the Player's Handbook even. I am looking at the "Ammunition, +1" and I cannot find it to add it to the character. So it's rather an issue of what the character sheet supports at the moment and not what is unlocked in order to do inventory management?

 1) have you purchased the PHB here or are you in a campaign with content sharing turned on? If the answer to both is no, your access will be limited to SRD/basic rules content for magic items

w)did you look for ammunition or a specific ammunition type like arrows?

Quote from SolFolango >>

I am unable to add items from the Player's Handbook even. I am looking at the "Ammunition, +1" and I cannot find it to add it to the character. So it's rather an issue of what the character sheet supports at the moment and not what is unlocked in order to do inventory management?

 Ammunition, +1 is in the Basic Rules, not the PHB. As such, it is freely accessible to everyone on DDB. But you will never be able to add “Ammunition, +1” to a character sheet because it isn’t an actual item, more a category of various items. You will need to add an “Arrow, +1”, or a “Crossbow Bolt, +1” since those are the specific items.

I hope that helps.

At 5th level, my monk selected Dragonhide Belt +1.  I can see it when I open FTD, but it's not available when I try to manage my inventory.  Help, anyone?

Quote from Shawn93 >>

At 5th level, my monk selected Dragonhide Belt +1.  I can see it when I open FTD, but it's not available when I try to manage my inventory.  Help, anyone?

 Do you own FTD, or do you have access through content sharing?  If the latter, then the character must be in the campaign in which the content is shared.  

If you own FTD here, try checking your entitlements--see link in my signature.

I can see the sword of answering and it’s types in basic DDB but I can’t add it to my paladin, either just sword of answering or the specific name that matches their alignment 

Quote from rdub213 >>

I can see the sword of answering and it’s types in basic DDB but I can’t add it to my paladin, either just sword of answering or the specific name that matches their alignment 

 
They are from the DMG. I will hazard a guess that you are in a campaign where content sharing has been enabled in which you do have a character - thus letting you see them - but the paladin you're trying to add these to is not in that specific campaign. This would explain why you can see stuff but not add it to the character. You can only add shared items to characters in the campaign where it is being shared.

I have the DMG myself and was able to add all forms of the sword of answering to my test paladin, so they are working fine.