ipod nano help

[n2dzny]

I am having trouble with my ipod....I have 286 songs in my itunes library, but only 206 show up on my ipod. I am not able to play all of the songs in my library because a pop-up comes up and says 'not able to find the original source...' also, I downloaded a song from itunes and it shows up on my ipod, but it will not play.
Please help!!!

I did look at the ipod help web site, but got nowhere.

TIA