iPhone 6 "waiting for items to copy" runs for hours

My iPhone is hanging with a "waiting for items to copy" message for hours ...

All the latest updates etc.

I see discussions over the years going back to iPhone 4s with this problem but not any recent ones...is there some fix I should be aware of?

I've let it run overnight and it is fixed in the morning only to return a day or two later when I do a regular sync.

Any Apple links etc to explain this?

Thanks

Tom