One thing you might try is dragging the songs from iTunes to the iPod, usually that will get them to copy over.
I've been seeing this a LOT latley since iTunes 12, and even more with 12.2. Artwork went missing on my iPhone, some albums don't sync right. Some albums don't even have the track numbers lined up. The iPhone will show songs 4-15 and say that there are 11 songs when it's really songs 1-12 and all sorts of messed up stuff.
That being said, my iPod classic has been perfect through it all.
Just yesterday my roommate went to sync his iPhone with his computer (different computer, different iPhone, same version of iTunes) and it absolutely would not sync the entire album he was copying over. His solution was to create a playlist of just that album and sync the playlist, and that worked.
I have had some missing on my iPod video though, and the only way to fix it for me was to remove all the music and re-sync the whole thing. That also fixed the artwork issue.
Apples software quality has been going downhill since 2014. I really hope they can pull it out with iOS 9 and El Capitan.