Youtube won't play after jumping forward

When I jump forward in a youtube video, it just freezes and no matter how many times I press the play/pause button (or just the video area) it doesn't start playing. It usually shows the correct screenshot for whichever part I jump to, and it indicates that it's playing (it shows a pause button in the lower left corner instead of a play button).
When it happens, the video won't play any part, not even if I jump back to the beginning. Reloading the page makes it play normally, though. Until I try skipping forward again.
It seems to not happen to all videos, but the ones I've tested have apparently all been html5, so it's not caused by the video being either flash or html5.