iBooks iCloud Storage Limit?

I have enabled iCloud storage in iBooks on my Mac(s), iPad and iPhone. I have been slowly importing PDF and ePub books into iBooks on my macbook again so that I have a central location to store them(and can access from iPad, etc).

Whenever I import though, iBooks gets about half way through processing the books I am importing and tells me that my iCloud storage is full. From that point on, it will tell me it is storing the books on the local device only.

The problem is, I have 200GB of iCloud space with only about 40GB used. 30GB or thereabouts would be books and the rest would be photos, backups, etc. i have checked from multiple devices and ensured my billing is ok(and it is, I definitely have 200GB space).

So my question is, does iBooks have a percentage limit of how much iCloud storage it can take up? Or maybe even a hard limit?

For context, I have tried system restarts, application restarts, different internet connections, etc. iBooks does not upload any additional books after the app is opened again unless I do another import, in which case it will get about 20-50% of the way through the files before it dies again.

I had this same problem - books added on my Macbook Pro stopped showing up on my iPad. I solved it by deselecting iBooks in iCloud on my Mac (System Preferences/iCloud/ ... click on Options tab beside iCloud Drive ... scroll down the list and deselect iBooks.app. If iBooks is open, a message should pop up that you can share books over the cloud, and it gives you the option to do that. Select that option. By deselecting and re-selecting in this way the books I was trying to share over the cloud immediately appeared on my iPad.

Apple menu/System Preferences/iCloud/iCloud Drive/... "options"... place a check mark in front of "iBooks"