Nothing's been changed within iTunes settings, so AFAIK sync should work just as before.

Try ejecting your iPod, shutting down iTunes & then restarting. Or even reboot your computer

Do you have automatic sync enabled or do you manually manage files & folders? I manually manage my files & folders (manual sync) just because with automatic sync there's always the chance of losing files from the iPod if changes are made to the iTunes library & then iPod is synced before you realise that you might've deleted some files from iTunes that you still want on your iPod.

Syncing Music to iPod