Q: I can’t play the albums I purchased from the iTunes Music Store on my iPod. All my imported tracks play just fine. Are they in a different format? How do I get my IPod to play all my music?
A: It sounds like your iPod might not be authorized to play your music. This is usually a transparent operation – iTunes recognizes that your computer is authorized, then copies the necessary information to the iPod.
Try selecting Advanced > Deauthorize Computer in iTunes. Then, select Advanced > Authorize Computer, and enter your Apple ID and password. This should reset the authorization.