Also different instances hide different communities and so they don’t show up at all unless you are a direct subscriber.

I learned about this on my instance when some news communities were hidden, turns out my instance has quite a list of hidden communities, which isn’t a bad thing, but it can contribute to the incomplete timeline issue and if you don’t know about the hidden communities feature its very hard to diagnose why you’re not seeing what you expect to see.

In my case I wasn’t subscribed to some of the news communities because I would catch the top posts in /all, which keeps my subscribed feed focused on things I’m actively interested in. So hidden communities are a challenge when you use /all and /subbed in that way.