So I think what you’re saying is that you’ve created a Lemmy instance and joined a community from another Lemmy instance but you can’t see all the posts?
My understanding is that your instance will only start syncing content from that community AFTER someone on your instance joins it. If you’re the first person on your instance to join cats then you won’t see anything from before the moment you joined, but you will get all the content that is posted after you joined. If you aren’t the first person on your instance to join then you’ll be able to see older posts.