It's most probably the immense load, I too had troubles finding a few communities on feddit.de that I knew existed on lemmy.ml, I had to try several times for them to show up.
I'm subscribed to them now but sometimes it's hard to see new posts or to comment.
It's a difficult situation, we're hugging all lemmy servers to death lol, we have to be patient and wait until things go back to normal, hopefully :D