help with ipod touch!

my itunes and ipod both have all my songs etc.on them but on my ipod info the 'sync entire music library' box is unticked. When i tick it, at the bottom it then removes lots of my songs videos so then i click revert changes as i dont want to lose loads of stuff. Although it says in a a popup that it will replace all my songs etc. with my iTunes library it doesn't as my library already has everything in it. This means I can't buy stuff on iTunes via my computer or import CDs (they go into my libraray but it won't let me sync it). Why is this happening and what do i do?