https://futurology.today/post/164245

Your instance only knows about users, posts, comments and even communities that it’s already been “told about”. This is how federation works in general on other Fediverse softwares too, but specifically on Lemmy it means someone from your instance needs to be subscribed to a community in order for new posts / comments to federate.

It is definitely a big downside for a lot of people, yes. I’ve seen various discussions among server admins of how to handle it, I think some use a bot to basically sub to everything as a workaround to populate the All feed without real users being subscribed. If your server has a meta community maybe you can ask around there whether they’re planning on anything like that?