My iPod refuses to sync some music downloaded from iTunes because my computer is "not authorized", although I have authorized it.

I have recently got a new computer, and decided to transfer my iTunes from my iPod (for I had to delete my music off my father's iTunes to conserve memory). I copied the files from my iPod onto my new iTunes account.When I synced my iPod with my iTunes, it refused to sync some songs which I had downloaded off iTunes onto my new iPod, for it says my computer is not authoraiazed, even though I have alredy authorized my computer. Wht should I do?