I have a fix to this issue. My scenario was as follows, iCloud photos switched on on the following devices, 1) MacBook 12" 2) iPhone 7 3) iPad Air 2. Up until holiday all 3 devices syncing perfectly. My problem is that I was on holidays for 3 weeks with no wifi, took lots of photos on iPhone 7, once returned back home and back on wifi, iPhone 7 has uploaded all pics to iCloud photos, Macbook has synced with the recent new holiday pics iPhone 7 uploads but the iPad Air 2 is not updating from iCloud photos since before I went on holidays so basically all the holidays pics are not on the iPad Air 2. I had this issue before on the iPad and fixed it with a restore and start from scratch, I did not want to do this again. Here is the steps I took to fix the sync issue without a restore.
1) Turned OFF iCloud Photo Library on iPad Air 2
2) Deleted ALL pics in camera roll so all folders showed 0 pics ( I strongly suggest backing up full iCloud Photo Library first, I did my full library from the iPhone as it had all pics via lightning cable to iMac via Image Capture App)
3) Restart iPad Air 2 holding sleep and home button until Apple logo
4) Turn on again iCloud Photo Library
All my pics including the holiday pics then started syncing to my iPad Air 2.
Hope this helps