Figured it out. Stopped the Lightroom CC app on my iPad, launched it again and sync began syncing again.
which iOS version are you on?
Did this occur once the device was in sleep mode?
iOS 11. Not sure about the device in sleep mode. I had loaded a large num of images, so the iPad probably had gone to sleep during processing of the images?
Same issue. Every time the iPad has been in sleep mode I have to restart Lightroom CC.