I have this issue using a MAC and iTunes 12 and an iPhone 6.
First time ever with an issue in iTunes and have always manually managed music. I am on my 5th iphone.
I always create playlists, (thats because I move entire concerts on and off of device) and then drag playlists onto the device. Snce a recent upgrade, (iTunes 12.7.3) the device only copies SOME of the music files, NEVER all of them.
When I try to drag files individually, they will NOT copy onto the ipod/phone device. We just see a grayed out, dotted circle next to the files that will not move.
I have re-started iphone/ipod device, restarted iTunes, re-started Mac, but the same thing happens. Any help is appreciated. All software is up to date.