Software needs to be updated
Download the latest version of the iTunes and then update the iPod.
iPod can't play the type of file
If you're not sure what file types iPod can play, click here.
Media file can't be found
Double-click the song in your library and show iTunes where it is.
Media file not authorized to play on your computer
Double-click the media file in your library and enter your account name and password.
Podcast size exceeds available space on your iPod
If your podcasts are set to autosync with your iPod and there is not enough available space left on the iPod for the current podcasts to transfer, your purchases will not transfer.
Turn off the option to automatically sync podcasts on your iPod and then re-sync your music.
Too many accounts
There is already content from five other iTunes Store accounts on the iPod.
The latest version of iTunes explains most of the reasons why some files aren't sent to the iPod. Get the latest version of iTunes here.
If you are getting no message explaining why specific content is not being sent to your iPod, you may need to reset warnings on your iPod.