My Artificer recently hit Level 8, and when I go to the Feat list in the character editor, the only one that shows up is Firearm Specialist. My DM has enabled content sharing and owns all the source books, so I should have access to far more options. 

Firstly make sure the character is in the campaign. They only have access to shared content if they are in the campaign with the shared content. Secondly - tell your DM to disable and re-enable content sharing to make sure the sharing data is up-to-date and refreshed. Thirdly - if that doesn't work - ask your DM to check their marketplace (https://www.dndbeyond.com/account/marketplace) to make sure their purchases are all synced up. Lastly - make sure your DM's subscription hasn't run out.

If neither of those work - I would get your DM to contact support via the link in the footer of the page.

Quote from Emmber >>

Firstly make sure the character is in the campaign. They only have access to shared content if they are in the campaign with the shared content.

 Appreciate the response, thank you! The character is in the campaign but is currently deactivated. Would that be the issue?

I don't know. Possibly?