Solved! Go to Solution.
I have the same problem. A certain file refuses to sync across (or Spotify on the phone refuses to play it). I have tried a handful of others, which worked fine.
The sync goes through the motions, and if I watch it when it gets to the problem file, the spinner goes round like it's loading, but the track stays grayed-out. I've made new playlists and I've updated old playlists to try to get the file across. The track plays fine on the Mac, and fine through the iPod app on the iPhone - I downloaded it through iTunes Match to test.
I've checked the Mac file, and it's not corrupt or otherwise showing any damage.