Audiobooks in Books not syncing to devices

Since using the new Book app, my audiobook library will not sync in full to my devices. 

Only a small portion of audiobooks will sync and some remain even though they’re deselected.

I’ve tried creating a Collection containing the whole library, but when synced across it only then contains the same small number of audiobooks. 

The small number on the devices are a mixture of Audible purchases, iTunes purchases and direct imports (as are the ones that won’t sync), so it’s not as if they are specifically iTunes-only purchases.

I frequently get the message “Some Audible files were not copied to the iPhone because you are not authorised to play them on this Mac.”

I’ve tried syncing All Audiobooks and Selected Audiobooks in the iTunes Sync Settings, but only the small number of audiobooks will sync across. 

Signing in and out of iCloud, turning devices on and off, reauthorising devices and double clicking to listen on each audiobook doesn’t help.