You can't downvote articles, you can't downvote replies to your own comments, you can't downvote comments that are a day or so old and older, etc. So make sure you're looking at a thing that's actually downvotable (e.g. a toplevel comment in a front page thread).
From time to time (like once or twice per month) HN goes to read only mode for a few minutes (while dang is making some weird update?), and the downvote buttons disappear. Can this be the explanation???
