clip won't play audiobooks

I downloaded an audiobook from Audible in MP3 format.

I’m working on a Mac - the book plays fine in itunes on the computer.

I have tried loading it into the audiobooks folder on the clip, but it simply shows as unknown and won’t play.  It does take up the memory, so I know it’s ‘there’.

I’ve changed the ‘media kind’ tag to audiobook, as suggested on another post.

I’ve also tried importing the book into the music folder, again as suggested on another post.  

I am of course using the latest firmware. 

Any other suggestions please?