I think I've figured out a way.
I had this problem and believe me, it's super frustrating. But somehow, the answer was incredibly simple.
This is what I did:
1. Connect device to iTunes.
2. Let it do its syncing business.
3. Once done, go to your playlist. If you haven't already, add all the music you want to sync into your device on this playlist.
4. Command+A (select all the items in the playlist).
5. Once you do, everything becomes highlighted right? Next to the names of the song, there should be a white circle with a blue-grey > .
6. Click one of them and it'll come up with a mini window. The first thing it should say is: Play Next.
7. Look for the 'Add To' selection on the window.
8. Press it, and it should list 'Devices' or 'Playlists'.
9. If your device is there, click it.
9. To make sure it works, check the top bar where it usually tells you how many syncing steps are next.
10. It should say three steps, and the first is (Waiting for sync to start) (Waiting for changes to be applied) and finally (Preparing to copy items).
11. It then should show you a progress bar of your songs copying into your device.
Hope this helped, I was so relieved when it worked. I had spent three hours trying to find a solution. 😀