It's possible the chapter numeration has been saved as follows:
1, 2, 3, 4....,10, 11, 12 etc.
You have to re-name chapters 1 - 9 as follows:
01, 02, 03, 04 etc.
Otherwise the drive in your car will read the mp3 files in the following order:
1, 10, 11, 12,...., 2, 20, 21 etc.