I have similar problem and I have seen other posts yet have not found and an answer. Anyone from Apple like to chime in? At best optimize is very inconsistent. I am paying for the extra iCloud storage, I don't mind paying I just mention that in case anyon questions iCloud space available - I have near 160GB of space in the Cloud. On my iPhone 4S 8.4.1 with iSettings > ICloud > Photos > iCloud Photo Library is on and Optimize Storage is "ON". However I am near capacity with about 60 MB of room left and it says my photos are 3.2 GB of space. When this happens I basically need to turn iCloud Photo Library to "OFF". Reboot. Wait a while then at some point I'll see "Preparing 13,000+ items " in iSettings > ICloud > Photos > iCloud Photo Library area of my iPhone. Sometimes it will say "preparing 9 iitems" Wait? What? Why is this so unreliable. Yes I am on Wifi. Then from time to time it will say "Verification Failed" with an "OK" as my only choice. I click "OK" and then it seems to keep on going and I'd down to "Preparing 11,000+ items " . Not meaning to sound like whiner but how come Google's photo backup works so much smoother and consistently?