Q: My fourth-generation iPod nano does not sync all of my purchases. I replaced my computer recently and have 3000+ listed in the purchased section of iTunes but only 170 on my nano. Everything is synced.
A: The most common cause of this problem is your computer not being properly authorized for your iTunes Store account, particularly when you’ve just replaced your computer. iTunes will only transfer purchased items to your iPod nano that your computer is actually authorized to play.
Normally attempting to play any purchased item in iTunes should pop up a dialog box asking you to enter your Apple ID and password so that iTunes can authorize your computer to play those tracks. You can also authorize your computer manually by choosing the Authorize This Computer option from the Store menu in iTunes.
Note that if you have content in your library purchased from more than one iTunes Store account you will need to authorize each account separately.