i've noticed this too, mostly with lemmy.ml communities. i was assuming it was because of their server being overloaded, and it looks like they have migrated to a new server now, but it still doesn't appear to be working. i will investigate some more.
are the communities you're having issues with mostly on the lemmy.ml instance? or others?
edit: just tried subscribing to a community from .ml from a different instance, and got the same issue. so i'm going to assume for now that it's an issue with their server.