Recently, quite a few of my purchases I got from iTunes weren't working on my iPod. I sent an email to iTunes, and the person who responded wrote this
This issue you're currently experiencing is actually very common with videos and 5th generation iPods. We believe it might be code-related in some of the files. Please be assured that Apple is currently working as fast as possible to come up with a resolution for this problem. Unfortunately, I'm unable to say exactly when this will be finished.