Q: iTunes won't sync new music onto iPhone 6 after iOS 8.4

What worked for me was turning on iTunes Wi-Fi Sync.

Sync your iPhone, iPad, and iPod touch with iTunes - Apple Support

Settings>General>iTunes Wi-Fi Sync>Sync Now

What worked for me was turning on iTunes Wi-Fi Sync.

Sync your iPhone, iPad, and iPod touch with iTunes - Apple Support

Settings>General>iTunes Wi-Fi Sync>Sync Now

Did you ever get a response?  I have the same problem each time I plug into itunes.  It says preparing to copy, then nothing happens.

Thanks!! It worked perfectly, and you help is much appreciated

Getting the same issue. Have not been able to sync last 200 songs or so since upgrading to iTunes 12.4.

Remember the good old days when everything just worked?

Make sure the music in your iTunes are still on the same hard-drive you loaded the music onto iTunes with, and that external hard-drives that you loaded music onto iTunes are switched on.

After going 3 times to the Genius bar and several calls to apple support, they weren't able to figure it out. So I decided to try on my own and just by luck I found that the solution for me was to uncheck every item in the Options box. I had Sync this Iphone over WI-FI and Sync only songs and videos, checked. Since I unchecked them on iTunes, my mac and my iPhone are synchronizing all my playlists with new songs perfectly. Hope this will help you.

This seems to have been my problem.  I seem to have multiple itune and music folders on my PC.  May have happened while recovering from a hard disk crash.