Lemmy federation, server to server, is having a number of problems. Comments and postings aren't always getting copied over to all the servers who have subscribers. For over a week, Lemmy.ml has had regular nginx 500 error codes on the front page, sometimes you have to refresh 3 or 4 times to get a page to load.
Growing pains.
When subscribing to communities it occasionally says “Subscribe Pending”
You should generally click the pending button to cancel the subscription request and then try again, possibly after waiting "a while" for the instance to have a moment where it feels less existential dread.
I have a couple pending subscription but they still show up in my subscribed list and I can interact with them fine so at least for me it’s just a visual bug.