Q: Audiobooks are not syncing to iBooks

Some of my Audio books are not syncing to iBooks.

Everything is up to date.

They are present and working in iTunes.

Books are from various sources (majority are Audible but not exclusively)

iTunes is authorized for Audible

I have ticked Sync Audiobooks and then selected audiobooks in iTunes.

The books I want to sync are ticked.

Some are going over and some are not.

There is plenty of free space on my iPhone .

Any suggestions welcome :-)