You generally seem to need to have at least 1 account from that instance subscribed to your magazine for federation to kick in (which then still can take a bit of time). Same works the other way around. The unfortunate thing is that you kinda have to use external community / magazine indexers to search for them, unless you know the exact name & instance domain they're in, since a search won't find non indexed ones by itself.
Imo one of the largest issues regarding the fediverse.