Given that most people in this thread complaining about their upvotes not registering seem to have a karma below 500 it might well be that. If that's the case I

Given that most people in this thread complaining about their upvotes not registering seem to have a karma below 500 it might well be that. If that's the case I'm surprised they didn't make it explicit by removing the arrow altogether (like the down arrow is not displayed when you don't have enough karma for it). Seems a bit deceitful. That being said it might just be a bug somewhere.