Why can't I upvote this post?

So I was trying to upvote this one post, and every time I tried to, when I would go back to it it would say I hadn't? So I upvoted if again, then went back, and then again it said I hadn't done it. I did that a few times. Then I went to another post, upvoted it, then I went back and it still said that I had upvoted it. So it's just the one post that's for some reason not letting me. What's up with that?