Why won't iTunes let me upload videos onto my iPod Touch?

Not all iTunes video can be transferred to iPod, please make sure your video is compatible with iPod. If the video dimension or video bit rate beyond iPod limitation, even though they are mp4 video, you can't upload them to iPod. For example, some iTunes purchased movie are High definition movie, the resolution is too high to play on iPod, you can't sync it to your iPod. For more information about iPod compatible video, you can visit apple official website.