@blag
This also happens to Mastodon accounts that your instance doesn't know about.

If no body on your instance is already following a particular account or it isn't coming in via a relay, then posts doesn't appear in the federated tab or in hashtag searchs. It also previous posts don't get seen when you first follow them.

Instances with a small amount of followers see a lot less posts than larger instances with lots of people following lots of posts.

You will see any new posts they put up from now on.

If you tap on the tusky screen under their account, it will take you to the web view of their instance directly, where you should be able to get everything they have posted