It’s actually working, but there is no recent activity on that community around the time you add that community, which is why it appear empty. To manually populate your instance with the older posts, try copy pasting the old individual posts you’d like to “pull” in the search page.
~~This is not working for me, I’m trying to subscribe to https://kbin.social/m/aspiememes but nothing is showing up. ~~
So you have to make sure the search is set to “All” and not “Community”
I noticed that too on my instance, but maybe 10 minutes after subscribing posts started federating properly.
It’s actually working, but there is no recent activity on that community around the time you add that community, which is why it appear empty. To manually populate your instance with the older posts, try copy pasting the old individual posts you’d like to “pull” in the search page.