We're still learning the ins and outs of Federation, but I think it's mostly just a matter of waiting. Here's what the official documentation has to say about it:

If you search for a community first time, 20 posts are fetched initially. Only if a least one user on your instance subscribes to the remote community, will the community send updates to your instance.

So Federation should continue to improve over time.

@astroturds

If you’re interested in a little more behind the scenes info on how this works, (and since I want to make a test post to see how it shows up in Lemmy):

Since there’s no central clearinghouse for content in the distributed Fediverse, each instance broadcasts its users’ new posts, but only to other instances that need to see that content, generally because they host at least one user interested in it.

So you’ll see times when your instance won’t have received any older content before its first user followed the remote account. After that, the remote instance knows to start sending content to your instance, to that user really, but then your instance knows about the content.

In other words, your instance begins its subscription to the remote account by having any user begin to follow it.

@ValueSubtracted