Initial sync of Photos from iPhone to iCloud Photo Library freezes

I had about 13,000 photos on a MacBook Pro (10.10.3) in iPhoto. Imported all to Photo, turned on Photo Library and they all transferred and synched to iCloud. Turned on iCloud Photo Library on two iPads (iOS 8.3) and one iPhone 6 and they synched just fine; all four devices now show the same set of photos and changes to any photo on any device are almost immediately reflected on all other devices. Very cool!. But my wife's iPhone 6 got about 15% of her photos synched to the cloud and then the upload/download process just stopped—that was about a week or so ago. I read Walt Mossberg's description of a similar problem he had (a "rare bug") when turning on iCloud Photo Library that required a special fix from Apple. Something to do with a photo having multiple copies on iCloud that caused the sync process to freeze. But I can't find any fixes for the problem except taking all photos off the phone and then synching, and then reloading the photos, but that sounds a bit scary.

Can anyone help? It's frustrating to have had the process work so well on four devices, only to have a fifth freeze. All are updated and all have similar settings.