I've posted this earlier but it is buried back a few pages, so here again is my kludge which works each time I complete the cycle, in this order. This is after only doing some odd uncheck/recheck sync music in general, but no changes to the file system, no deleting of files, no other work with playlists, no resetting the phone to clear any music or settings, yadda yadda yadda...
1) Connect the iPhone 6 using the Lightning cable, and it is set to ON the Summary page to "Manually Manage Music and Videos" because you left it that way when you disconnected after step 3 below (like I said, it's a complete cycle). Apps will sync, some music play information will go from the iPhone to the computer. No music will sync from the PC to the iPhone.
2) Click the "Manually Manage Music and Videos" to Uncheck it, turn that OFF. Apply/Sync this setting. Some tasks flash on the progress bar, but nothing really happens here. (this is ultimately the failure point of the current versions). You can make changes to selected Podcast, Video, and Music playlists at this point. iTunes will remember them in the next step.
3) Set the "Manually Manage Music and Videos" to ON, check the box. Apply/Sync. This should start a regular sync, should copy over real music files to your iPhone. It may be able to just update songs, run a subset over to complete those not on the phone, it may need to go through the entire list of songs because they've been lost from the phone, but eventually, for me, this third step has gotten songs that work on to the iPhone every time.
Leave the "Manually Manage Music and Videos" checked ON when you disconnect (I click the little "Disconnect" icon just to be OC) so it is at that setting when you come back to connect again.
It really is counter-intuitive that Manually Managing applies the playlists you want onto the phone, but we are currently in an alternate reality here: It's a new Apple Zen thing that Steve Jobs is killing us with, for making the iPhone too large.
