This usually means that no one else on this instance subscribes to them. The best way to subscribe to a community is to search for it then subscribe. Joining a community, currently, only pulls the last 20 posts. However, if the posts isn't present that you wish to comment on you can search for the URL of the post.
Thanks, they are all showing up now. Previously they weren't showing up in search either, so there wasn't a way for me to subscribe to them; from discuss.online's point of view it seemed as if they simply didn't exist. Maybe after searching for them / trying to visit them, that triggered something in the backend to pull them in?