easiest way i have found to resolve the issue is go into itunes while phone is plugged in. you will have a list of tabs across, summery, info, ringtones, music. select music and where it displays sync music unselect that and hit apply in the bottom right (dont worry your music should be saved on the computer) once the sync is complete re check the music sync option. ive noticed this to work 95% of the time......hope this helps