Just bought about 10 songs off iTunes (stranger than fiction soundtrack, HIGHLY recommended). All songs play fine on my computer, but three of them won’t begin playing on the iPod - they show up as being on the iPod, but click play and it just passes them over.
Have you updated the software on your iPod recently? Sometimes I have a devil of a time trying to figure it out after the software gets updated. I have an older iPod (the 4GB Mini) so stuff like this happens to me from time to time.
iTunes just recently updated to a new higher fidelity form some of it’s music files. It may be that your Nano won’t play those, although I doubt it. I assume you have tried to re-sync your Nano?