If you mean you can see the community but there are no (or very little) posts, look in the side-bar for the local subscribers stat. If it’s 0, that’s why. If someone subscribes, new content will federate.
It would be nice if there was a tool to find these communities that’d be useful for smaller instances, but I wouldn’t know if there is one.
Why can’t I see some communities
My first guess would be that your instance isn’t federated with them
If you mean you can see the community but there are no (or very little) posts, look in the side-bar for the local subscribers stat. If it’s 0, that’s why. If someone subscribes, new content will federate.
It would be nice if there was a tool to find these communities that’d be useful for smaller instances, but I wouldn’t know if there is one.