I fixed this problem for myself:
Go into your iTunes music
Make sure all songs are checked
If not, For Mac: Command A, "right click", check selection
For PC: Control A, "right click", check selection
After you select all songs, click on the button on right upper corner to go to your device
Click the Music tab
Select the all music option, or whatever may the case be
Resync (transfer your purchases from ipod to itunes to make sure you dont lose anything first)
Hope this helps