
Finally I de-authorised and re-authorised my iMac (I have 4 out of 5 authorisations). I have also ensured that the setting to sync collections across devices on the Mac Books App is selected. deleted them from my iPad Pro and re-synced), restarted the iPad Pro and uninstalled and re-installed the Books App on the iPad Pro.

So far I have re-synced the audiobooks (i.e. When I try to play the "unplayable" audiobooks on that folder they play! Just for information I have an old iPad Air which syncs with my Mac Air (Mojave) via the old iTunes App and all audiobooks sync fine and play fine. To make things even stranger I have used iExplorer to sync my audiobook collection back from the iPad Pro to a folder on my iMac as backup. What makes things strange is that the very audiobooks that will not play on the iPad Pro do play successfully on my iPhone XR. Audiobooks purchased from the iTunes Store seem to be fine. You can watch live video footage and communicate in real time with your garage through the myQ app.

If there is a common factor it appears the audiobooks that will not play are mp3 files I created from my CD Audiobooks collection and transferred to my Books App on the Mac. It includes a built-in 130 wide-angle camera, LED lighting, and two-way audio for complete surveillance of your garage from your phone. The audiobook image is on the iPad Books App, but will not play. When I sync my iPad Pro to my iMac (Big Sur) some of the audiobooks do not play on the iPad.
