iTunes Library not synching to iPod

I connect my iPod to my computer, my main original computer that is authorized and nothing happens. there is more music in the iTunes library than on the iPod. I have set to auto synch. I have gone to music tab and indicated synch all music, videos and art. there is plenty of room on the iPod but iTunes does not even attempt a synch. I can see the iPod in my device list. I am on a PC running windows XP