Found this on another site on which the same problem was reported. In my case, it turned out that my .mp4 files had a higher resolution than the iPod could support. It may be that you (and I) will need to convert our mp4 files to conform to the iPod's abilities, using video editing software in those cases in which the iTunes menu 'File-->Create New Version-->Create iPod or iPhone Version' doesn't do the job.
"The iPod can play MPEG-4 video to the following specifications (I quote):
'up to 2.5 Mbps, 640 x 480, 30 frames per sec., Simple Profile with AAC-LC audio up to 160 kbps, 48 Khz, stereo audio in .m4v, .mp4, and .mov file formats'
Files which fall outside these specs evidently won't work."