When you subscribe to a community, you only start receiving data from that point on. It doesn't go and download the history, that'd be very resource intensive on the remote instance. Thus it fetches a handful of posts, and no comments as a preview of the community, and after a couple days of activity is looks mostly normal.
You should have all new posts and comments coming in, comments here mean it's probably working. Sort by new and things should start coming in. If you don't see any new posts or it stays stuck on subscribe pending, it means there is something probably wrong