iPod Set Poster Frame

I am not able to set the poster frame for videos I added to my iTunes library within the last few days (but other videos work fine). These videos are MOV format, and added from my hard drive, not from iTMS. Only the new videos have problems. Does anyone know what is going on?

Bump...I still can't set the poster frame for those videos, but other videos I just added work fine. Of the videos that won't work, three of them are 240x180 @ 24 fps and the other is 320x240 @ 30 fps. All the ones that I can set poster frame on are 320x240 @ 24 fps. Has anyone else had trouble setting poster frames?