Can't sync QuickTime movies from PC to iPad 3

Some of my QuickTime (.MOV) movies sync, some don't.

The movies are made with a Canon S100 camera, then copied from the SD card to my PC (direct file copy, no import software). From there I drag & drop the movie files to iTunes. I also have tried File ---> Import in iTunes.

Some of the movies appear in the iPad right after synbcing and some of them never appear.

I am running Windows 7 64 bit with iTunes 10.6.1.7 and iOS 5.1.1 on the iPad.

There is plenty of storage on the iPad, so space or file size are not problems.

They are all .MOV QuickTime files - though some are 640x480, 720p, 1080p, etc.

Any suggestions?