phone 6plus syncing problem

I am on my 3rd 6plus 128gb iphone and continue to have the same problem. Iphone will not sync music consistently and many times will stay in sync mode when attached to the computer and never update. I have spoken to numerous senior tech engineers at Apple and genius bar techs with no solution. I also bought new Mac and made that primary computer with no success. I have had every model of Iphone and never had this problem before. Rumors of software issues with the 128gb 6+ chip is the what may be the problem. Help?!