Sometimes I think you can get the equivalent of a time-out error just as you post the vote, and it doesn't "take" so it comes back again. I occasionally click on a vote and get taken back to the thumbnails page, and I think that vote doesn't register either. Otherwise, I don't know -- try to keep track of which images are affected, and maybe which ones come just before and after, and report to the admins (or repost here) -- I'm sure more specifics would help find the problem.
While making a quick browse through the thumbnails for the 'Parts' challenge I came across a page that was identical to a page I had looked at a few pages back -- thumbnail pages that is. The thumbs were in the same order as on the previous page, just repeated. When I went back to the previous page the images were some I had not seen before -- meaning I had not yet made it thru enough pages.
Normally a thumbnail page will remain the same when I browse thru them. The above is certainly not the expected behaviour based on past experience. :p
