I have anew iPad Pro with 512GB memory - so plenty of space!

My previous 3rd Generation iPad (sadly deceased after falling into a river with me) sync'd 100% of my Photos.

However, when I sync the new iPad (via USB cable using iTunes) only approximately 8,200 of my 10,700 photos arrive on the iPad.

I have, on the relevant iTunes/iPad page clicked Sync all Photos and Albums, but no matter how often i delete and start again, or change and reset the sync criteria, the same result ensues. I can see nothing about the "missing" photos which might indocate the cause although, of course, i haven't checked roughly 2,500 pictures to try to identify a common theme!

All music and other sync items go across in full as expected, it is only Photos which is failing.

I have posted this question on both the Apple Forums and iPad forums and neither crie de coure has resulted in a single response!

has anyone else had a similar experience, or can anyone suggest a cause and/or fix?

iPad Pro 10.5" 512GB under iOS 10.3.3 (14G60); iMac under macOS 10.12.6 (16G29); iTunes, Photos 2.0 (3161.4.140)