iPhone can't finish sync, but I can backup and upload photos
Our iPhone 6+ can't finish sync, but it can finish a backup and download photos.
I support all our family's iPhones on my iTunes. I can take care of all but one, an iPhone 6 Plus running iOS 11.2.6. I connect to an iMac (2017, 27in, MacOS 10.13.3). I use iTunes 12.7.3. I have tried methods recommended in this forum including power both on/off, reset, restore, sync over wifi/no sync over wifi, change cables, and transferring all purchases. iTunes goes through the various sync stages and then gets stuck on the "waiting for sync" last step. Then the window pops up with failed to finish sync.
The biggest impact means that no photos from our shared Photos files are loaded on the iPhone 6+. Why can I backup and download Photos, but can't sync iTunes? What other actions can I take?
S/ OB94
iPhone 6 Plus, iOS 8.4