The movies on my iPhone don't show a preview icon They used to show a preview, but don't show anymore on my iPhone or iPad (synched to the same library). The videos under the Home Videos tab appear with normal thumbnails, but the Movies tab are just gray film strips.

Is there a way to fix this or trigger iTunes to regenerate thumbnails?
I've had that happen to me, the only workaround was to turn off Movie sync'ing in iTunes, Sync your device, then turn Movie sync'ing back on, Sync again.