Cannot sync all of my books from iphone to ipad

I'm trying to sync all of my ibooks from my iPhone 6s to my iPad Pro, but for some reason I'm getting only partial of them. I checked the settings on both devices and iCloud Drive is on and iBooks is checked on, just everything seems fine to me. I restart both devices and no luck, still having less books on my iPad then the iPhone. Both devices are operating the latest ios 11.2