Q: Why cant i sync my episodes of sherlock from my mac to my ipod?

Was the iPod previous synced to another iTunes library/computer?

Have you succesfuly synced from this iTunes libary/computer before?

Do they play in iTunes?

Do you have the right boxes checked to sync?

iTunes: Syncing media content to iOS devices and iPod

Then yo must have 3G or earlier iPod. Those can only play/sync SD videos.

iTunes: May be unable to transfer videos to iPhone, iPad, or iPod

iTunes: Frequently asked questions about viewing and syncing videos

Try redownloading the SD version by:

Downloading past purchases from the App Store, iBookstore, and iTunes Store

and then syncing that version.