NSUbiquitousKeyValueStore not syncing with iCloud

I had the same problem. Logging off and back on my iCloud account on the device solved it for me, too.

This problem seems to appear when you have installed the app with the same bundle identifier on the device before you enabled iCloud entitlements. I had this problem on 2 testing devices. A third device on which i installed the app the first time with iCloud enabled, made no problems.

I have had the same problem, thanks to this answer I was able to figure out, however all I did was uninstall app -> restart device -> install app rather than sign in and out of iCloud.