If the communities are on instances with Lemmy 0.17.x and older, the join function doesn't work properly. You have already joined (and will see updates in your feed) and can interact with it.
Joining really doesn't mean "ask for approval", it means "subscribing to their feed". There are no private communities and commenting restrictions.
Another issue could be that the instance was offline while you tried to subscribe, which leads to a missing ping back, but again you are subscribed regardless.
Joining communities sucks in many apps. Sync does a great job here; but if indeed the instances are not federated with the ones you are using in the first place, that will lead to issues.
One of the reasons why I created a backup account with infosec.pub, which is federated with just about anyone.
I can't tell, I have a youtube account.