Problem with fullscreen

Hey everybody.

I'm new to this forum, but i hope some of you might be able to help me. When I try to play some my .m4v-files on the XBMC, the picture doesn't show on fullscreen mode. But when I go back to the menu and the video is just running in the background, it's running perfectly. I've got the same problem when i play on of my video add-ons. I've got absolutely no idea why this is.
I desperately need some help