The issue lies with lemmy.world who are still on lemmy version 0.18.5. If there is any downtime or an issue with their server (of which historically there has been a lot of) then federation doesn't work properly and the server won't push new comments or posts to other servers. There can also be some discrepancies based on when someone subscribes too, although I don't know if that's been fixed now.
You can see this happens on other servers too:
The newest version of lemmy (0.19) contains a persistent federation queue so federation issues from between 0.19 servers should be massively reduced.
Idk why lemmy.world hasn't updated either, but it would be a benefit to the lemmyverse for them to update soon.