iTunes disabled songs from playing through my iPod.

When I got my new Macbook Pro I synched my 5th generation iPod Touch (16GB) to get the music from my iPod to also be in my iTunes library on my new computer. I had around 1100 songs on my iPod and only about 600 transferred to my computer. The songs that didn't transfer to my computer are still on my iPod, but are greyed out and when I go to play one, my iPod will just skip the track completely. It seems as most of the songs that aren't playing are the ones that have come from CDs that I've downloaded onto my iPod through iTunes.