iPod Touch 4G won't sync... Help!

Please help! I have an iPod 4G, and I connected it to my laptop. It transferred all of my purchases onto my iTunes library for my laptop, but now it won't let me put other music from my library on the iPod. It allowed my sister's iPod (same kind) to sync just fine, but now it's saying that mine cannot be synced. VERY frustrating. I thought maybe I need to update my iPod, but then it will delete everything. Someone please please PLEASE help.

Thanks,

Madi