Check out section 2 of the find and subscribe section of https://lemmy.blahaj.zone/post/61827.
Notably, searching the "community" type for the fully specified community name returns nothing. You have to search the "all" type. I have no idea why this is the case, it makes no sense. But if jerboa is searching the "community", which would make sense for that searchbox... you'd expect no results to be returned.
If your instance knows about the community, a keyword search will probably work, using just the name part of the community without the bang and server bits.