Mac and iPod up to date. iPod playlist songs show on Mac, but while playlist names are on iPod, the music ISN'T.

That's it. I've got Yosemite, and on the iPod "G4" 6.1.6.

I made some changes to my playlists, with some new playlists, and synced.

The computer shows everything good. But on the Touch, the playlist names are there, but no music.

What am I doing wrong?

Thank you.