iTunes won't sync music library back to iPhone 6 or 5
Many users have reported this problem with syncing music on iOS 8. The following steps can be taken to Sync the music back onto your iPhone.

• Uncheck “sync music” in iTunes

• Delete Music (General > Usage > Manage Storage)

• Re-check “sync music”

• Sync

I did this and also had to make sure, under the Summary tab in iTunes that "Sync only checked songs &amp; videos" AND "Manually manage music &amp; videos" were checked. ONLY then did it sync successfully.