That same thing happened to me. Should be an easy fix. Sometimes the artist name is different. Perfect example for me: some of my songs for Three days Grace are under 3 Days Grace, so it shows up as two seperate artists. or there is no artist name and you would go all the way to the bottom where it says unknown artist.
But im saying in my library on itunes they are all correct in artist. Like before the last sync all of my immortal technique songs (7) were under immortal technique. After the sync, i check immortal technique and only 1 song shows up???
Im about to do another sync so ill see what happens...