Books Not Syncing

I often add epub files to my Books library on my iphone. I would like these files--not purchased through the iBook store--to sync across my devices but they do not appear on my Mac or iPad. I have enough iCloud storage to accommodate them and have Books turned on for iCloud sync. I also have the "automatically download new purchases" option selected in Books. Despite this, dozens of epub files I've added to my phone manually don't upload to iCloud or sync. Would love it if someone could tell me how to fix this. Thanks!