Try adding them manually. Connect your iPod > right click on your iPod in iTunes > iPod options > click the iPod tab > choose to manually manage songs & playlists. Now just drag from iTunes into your iPod.

If that doesn't work then sorry...I don't know Unless there's some sort of DRM associated with the files??

Have a look on the iPod forum, you might find more info over there!
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Some songs in iTunes won't copy to iPod