I’m not going to be able to help you directly; the forum has a large number of posts on topic, though, so a search whilst you’re waiting for somebody more knowledgeable than I to respond may help you along.
Does this mean you cleaned the sync store and reuploaded? If so, I suggest seeing this post.
Do you have enough available iCloud storage?
No!
A cloud provider. As with every other sync option, you will find posts in the forum from users having trouble with CloudMe. I have found it to be very reliable (it has phases where it produces error messages in the log, bad gateways for example; these always solve themselves given a little time).
My feeling is that CloudKit is more reliable than iCloud (legacy) is; but no, I don’t think it is universally reliable. But I’m also not sure that is your problem here.