This appears to be one of the most limiting factors in Lemmy right now and is not unique to Mlem. Essentially, your instance can only “see” communities that at least one user on that instance has subscribed to.

Example: I’m on the lemmy.blahaj.zone instance. I’m not subscribed to !196@lemmy.world, but someone on lemmy.blahaj.zone IS, thus it shows up in my All feed:

lemmy.directory is trying to solve this by subscribing to as many communities as they can find, serving as an “/r/All” for Lemmy as a whole. But you can’t make an account there, so the idea is you browse the directory and subscribe to any communities you like, thus making them available to other users on your instance.

Hmm whenever I switch my feeds to “all posts” I’m pretty sure I can see everything on Mlem. I’m honestly not sure what the issue could be that makes you not able to see everything since it’s all pretty new to me too.

Mine seems to be showing posts from other instances, but I'm not sure what I did to achieve this. Have you, while logged into your local instance, subscribed to communities on another instance yet? Maybe that has something to do with it.

This is confusing me as well. Where do I go for a search to uncover new sub whatever they are called and then how do I change to only see posts from that sub? Using Mlem as well and as far as it goes it seems to be ok, just missing the filter and search options for now.

It's the downside of federation. It would be very hard to implement and come at humongous costs/traffic.