Yes, same for me, F11 full screen works if I do it before I start the slideshow, but not once I have clicked on play, even if I pause slideshow F11 has no effect. I have to put mouse cursor at top of screen to get the top taskbar to show so that I can 'escape' from the slideshow to another tab then F11 will work again.
You may need that on some keyboards and some laptops. But I don't think that is the problem. Like you once the slideshow starts running F11 doesn't work. This is on Opera and Firefox. More worryingly I started the slideshow in Opera in fullscreen and then couldn't use escape or F11 to revert without clicking to change to a different part of the website. It seems to be the wway the site is designed.