how do I transfer my music to my new ipod nano? I've keep syncing it but the music doesn't appear!

I've just replaced my iPod nano 4th generation with a iPod 6th generation, I'm trying to transfer my itunes library to the new ipod but it keeps saying "these songs can't be played on this ipod" what on earth does that mean, shouldn't the music be transferable no matter what?! Please help!