If your instance is not big, then communities from other instances may not be indexed yet, as it is triggered by local users’ research and subscriptions, and it seems to not work well if the target instance is overloaded. (as far as I understand for now) I’ve been trying to search for tiny communities that exist on lemmy.world, and after 24h, the research still doesn’t find them. It’s quite frustrating, and I think it hinders the development of new instances, which is problematic.
Edit: you have to search the exact URL of the community, I was only using the name, with the URL it works after a few tries.