The parent post could have been made "dead". That will remove the ability to do votes, etc, even from your threads page.
You probably already voted.
What happened is that I voted up a new comment and its vote become 2. But after I refreshed the page, its vote went back to 1. I tries again and the vote was still 1 and the vote button was gone. So I cannot vote it any more.
I think you are right that the button disappears after I vote. But somehow the vote was not counted.