I have an Touch iPod. I downloaded Dexter season 6. iTunes is telling me it is in the wrong format to play on my Touch. Seasons 1-5 worked fine. Did that change video formats that has obsoleted my Touch? All hardware and QuickTime is up to date.
If you downloaded HD format rather than SD, it will not play on older iPod touch.
You should be able to download the SD version by:
Downloading past purchases from the App Store, iBookstore, and iTunes Store
The sync to your iPod
iTunes: May be unable to transfer videos to iPhone, iPad, or iPod
iTunes: Frequently asked questions about viewing and syncing videos
iTunes: Sync purchased HD videos to iPod or iOS device
Retrieving data ...