If you haven't blocked anyone, any communities or instances then check you have both English and Undefined selected in language settings (along with other languages you want to see posts/comments in).
Ah! I think I'd blocked the community so it didn't show up in my All feed, so that's why the comments aren't being displayed although weirdly the post itself is. Is there a way to stop it showing up on All but still are comments of a post?
Cool. I don't know if there is a way to do what you asked. You might be able to do something similar by subscribing to all the communities you want to keep in touch with so that those communities show up in Subscribed/Home. That way All will be unfiltered and you can check it out every now and then to see if there's anything new, while focusing on the stuff you like in Subscribed/Home.