I am trying Lemmy for the first time, and have subscribed to a community on another instance as a test (https://sopuli.xyz/c/homebrewing).
I can see the posts come though, but few if any of the comments are there. Obviously if I visit the community directly they show up, but then I can’t post/comment as my account is with this instance.
One thing to note is that posts are by default ranked by “Active”. When someone comments, it’s back to the top (is how I think it works). If you’re after new content, you can change this to sort by new. If you are on the All tab and choose to sort by “New”, you should get a pretty steady stream of new posts.