Not all iTunes videos shown in ATV2

Hello

Now I have ATV2, I added all my videos into iTunes to have it streamed to ATV2. Unfortunately only 18 videos out of 31 are shown in ATV2.

I checked and there is no clear indication. Files being showed and not are .mov, .m4v or mp4, all of them can be viewed in iTunes on the iMac. Some of them are from a DVD, some of them are made myself. All of them are in the same network folder.

I also already plugged of ATV2 and plugged power back again in order to have a cold boot. But this did not resolve the issue either.

Any ideas? Do I need to dig into which kind of codec is behind the container of the video? Must it all be only h264?

Thanks for hints

Cheers

Michael