Sync help please!!

Hello,
I have a 120GB ipod classic and my son has a n 8gb nano. he was using a laptop for his itunes but that is playing up so I set up a multiple library on my PC for us. However, his nano will not download the new tracks on his library. It is syncing properly but not updating. I tried the old laptop and it worked so the ipod is ok.
I had to download itunes 9, which is where the problems began. I`ve ticked and unticked every sync box there is and short of taking a large hammer I`m stuck about what to do.
Can anyone offer any suggestions

Thanks