I've never seen an upvote fail. It's only downvotes. I know that sometimes, if I downvote, nothing happens, I reload the page, nothing happens, but if I click through to the direct link for the comment the vote has taken effect --- in other words, I know to check for that. What I'm seeing is, most of my downvotes don't count.<p>(Finding a random comment to downvote is more effort than I want to spend on this problem).