As for mastodon you have to search communities with their name but replace ! by @ as if it was a username (!asklemmy@lemmy.ml becomes @asklemmy@lemmy.ml).

I think that it has to do with lemmmy and instances. When no one has searched for a specific community from your home instance this community isn’t known by your instance. To find niche community you must use the exact name of this community (like !asklemmy@lemmy.ml for exemple) to find thoses exact names ans brosse all communities you can use lemmyverse.net