Do you see the content of these communities when logged out (ex: incognito mode on your browser) or through another instance / the home instance of that community?
One possibility is that the instance you’re on “deleted” the remote community, and it won’t sync anymore until restored.
We had to do this with a couple of unmoderated communities filled with spam, with an unresponsive instance admin. It was that or defederate completely from that instance, which I’d rather not do.