fourMaybe it's a problem with flash, try enabling the youtube html5 test and go fullscreen on a video that has no adds like this one watch?v=Hp7hxjo8jtU (first video I found, I don't know what it is). You can see if the video uses html5 if you rightclick the player. If it works with html5 maybe you need to re-install flash.
I tried it before, it looked like shit, then I tried it with the html5 and it went fullscreen. So I guess my problem is flash