Lemmy instances need to be made aware of communities on other instances (that is, bookwormstory.social doesn't automatically know about every community in the "threadiverse").
An instance can be told about a specific community by searching its URL in the search bar (on the website at least). This needs to be done once per instance in order to browse it from that instance.
If it isn't already, this would be a nice feature for Boost to do automagically.