Could be a couple of things. When did you subscribe to the lemm.ee one from your .world account?

If you're the first person from your instance to sub to a remote community, it won't pull in literally all past activity. I think it's supposed to sync the last 20 posts but in reality it seems to vary, and comments are anyone's guess. In theory though you should be seeing all posts and comments made after your subscribed.

One workaround for this if you need to reply to older stuff is you can search for specific post or comment URLs on your own instance to force a manual sync. Comments work best because they'll pull any parent comments plus the post itself at the same time and save you some work.

Now, all that said, it could just be a bog standard federation issue. It seemed like we were past the worst of that with the recent updates, but I've noticed some serious problems again in the last day or two. In which case it seems like our only option is to hope someone clever can figure it out and fix it.