I subscribed to a community, but I don't see any posts. They're visible on other instances.

Lemmy only starts fetching posts and comments from a community when at least one user on our Lemmy instance is subscribed to that community. Lemmy also has no ability to backfill old posts and comments. So, the most common cause of this issue is simply that you were the first person on our instance to subscribe to a particular community. New posts and comments should start arriving once you subscribe.

We have a user account that subscribes to the most popular communities to improve the user experience in this regard, but this scenario is still likely with many smaller communities. As our Lemmy userbase grows, the chances that someone will have already subscribed to the community you're interested in will continue to improve.