I had the same problem with syncing Music to my new iPhone 6. Syncing to my older phones was without a problem, but only with the new iPhone. My problem was that only purchased music from iTunes Store could not be synced with my new iPhone, although I registered my iPhone with my Apple ID (in the iTunes & Store App under iPhone's Settings). Other music could be synced without problem. The odd thing was that I could even sync music belonging to Apple ID of my family members (our household have family sharing), but not music belonging to my Apple ID.
My solution was logging out of my Apple account and logging in with the Apple ID of a family member on the iTunes & App Store (in the iPhone settings). Syncing with that other Apple ID went well. Afterwards, I logged back with my Apple ID and tried to sync my music, and it went well without problems.
Now, my music is syncing well under my Apple ID.
I hope that my solution could help some who were in the similar situation.
Best regards,
Aleksandar
My solution was logging out of my Apple account and logging in with the Apple ID of a family member on the iTunes & App Store (in the iPhone settings). Syncing with that other Apple ID went well. Afterwards, I logged back with my Apple ID and tried to sync my music, and it went well without problems.
Now, my music is syncing well under my Apple ID.
I hope that my solution could help some who were in the similar situation.
Best regards,
Aleksandar