Hi Sheri1
I had exactly this problem when using audiobooks. I spent ages communicating with Lexus via the dealer and it was obvious they had no idea how to solve it.
After numerous hours playing and reordering and experimenting and .....Zzzzz!
My solution, which works perfectly for me (65 plate), was to write the files to the USB stick in the order I wanted them played. The unit plays them back in that order. It is, admittedly, a tedious process, but it worked. Adding 65 chapters of a book individually is daunting but I couldn't find any other way.
Hope this helps
Rob