I keep approx 55GB's of offline music on my phone. And whenever I'm syncing it for the first time to the phone (like after a restore), iTunes most of the times finishes the sync, yet doesn't actually sync all tracks. When I look at the music list inside the phone within iTunes, the tracks not synced are greyed out. I hit sync again and iTunes finishes it in the second go, most of the time. Does this happen to you? It's not a new issue, it happened with all iOS releases I can think of and with multiple models of iPhone and I still cannot figure out what causes it.