I am able to full screen the Vimeo but not the Youtube even though the "full screen value" is still set to "true". From all these responses there seems to be a wide variance in experience. I placed a reply of mine from a "same, same" thread later in this one outlining where my videos "work" and where not. If you want to look for it.
I think it may be connected to similar issues that were outlined in this discussion. hope that helps