No matter which character it is, there are certain items that just do not appear when I try to add to the characters' inventories.
Everything is checked off and there are no sharing issues - I own the content personally.

For example, if I want to add the Denomomicon or Ruinstone of Mindguard Crown, they do not appear in the search when I try to add them to my inventory.

Follow this link and make sure that your Account Entitlements are in sync: (https://www.dndbeyond.com/account/marketplace).

That worked for some of my characters, but not all of them.

Check it again and then refresh the characters. If that doesn’t work then post again with the URLs for the affected characters so a mod can look into it for you.

Yea, it didn't work.

https://www.dndbeyond.com/characters/107808601
This is the character I'm trying to use it on.
Wanting to have the Mindguard Crown and Ruinstone, but, for some reason, they won't pop up for this character.

Even when I try to make a new character, the items are not visible.

And you’re absolutely sure you have access to the Phandelver and Bellow and Divine Contention adventures?

Yep. It's in my marketplace order history and everything.