I have all the books in dndbeyond shared with me (or I bought them). I am not able to add a tasha spell to my homebrew item even if I can clearly see that spell when playing in a campaign. 

Another thing I noticed, is that if I create a character in the campaign that has shared data in I get everything, but if I just create my own character it does not. I am not sure why that is as we can just move characters around between campaigns anyway.

Content sharing only works in the campaign in which the content is specifically being shared. That’s how it has always been.

Having content shared with you is not enough to unlock that content for use with Homebrew. Only the free content and content you have specifically purchased is available for Homebrew, content being shared with you is not.

Sounds good. that is what I thought, but I want to create the item for that specific campaign that it has everything shared in. Maybe I should go through my character sheet to create it rather than from the homebrew tab.

Or whoever owns that content can crate that Magic Item. Or you can just purchase the one spell you need a la carte and create what you want.