I’ve seen this mentioned a few times already: people wondering why certain communities don’t show up in their instance. You need to first search for that community via the magnifying glass icon and type in “!communityname@instance.domain”
Notice the exclamation mark at the beginning. Might take a few seconds. This will teach your instance about the remote community.
100% agree. People keep posting communities but manually having to watch for each one is tedious when you’re trying to add a bunch at once