Newly added home videos don't play

I previously added some videos to my library (.m4v MPEG-4 video files) and they play fine, but files of the same type that I add now do not play - they open to the first frame but do not play. No errors are reported. All files use the same H.264 video codec and I can play the files in Windows Media Player.

I re-installed iTunes and imported files again, but get the same results.

Any ideas?

Thanks,

GTGleeson