There's a bit of an issue with federation in general at the moment, it's being worked on. But also if you were first from your instance to subscribe to this community, I believe (feel free to correct, someone more knowledgeable than I) it now starts pulling in new posts but not necessarily going back and pulling in old ones.
One thing you can do is pull posts or comments in manually. If there's a particular comment you want to reply to, search for it's original URL on your home instance and it'll not only pull over that comment but also the post itself. If you just want the post to show up and don't care about any comments, search the URL for that instead.
Not ideal behaviour but we just need to work within the limitations for now.