Music won't sync to my iPhone
I recently bought a iPhone 6plus. I choose one with 128Gb of storage because I wanted to put my whole music library on the device. (about 65Gb of music) When I first synced with my computer (MacBook Air) it transferred most of my music, but not all. I then created a few playlists and they initially synced with the phone, but if I changed them on the computer the next time I synced they didn't update.
I then called Apple Support and the only solution that was suggested was to take all music off the device and start again. Several hours later that had worked all music and movies on the device.
Recently I bought a few new albums on my computer tried to sync on the phone and some different movies and nothing new came across, new music not on the device, change movies did not come across.
Can anyone help with a solution?
iPhone 6 Plus, iOS 8.4