Sorry for the delayed reply on this. I can't think of a good reason why this wouldn't work but have a list of things to try. If it doesn't work after this then we'll replace the hardware
- If you attach the motors to the battery directly, do they turn?
- Do you have any kind of bench top power supply that you could use to power the mini driver instead of the batteries to see if that helps?
- Does the blink sketch work on the mini driver?
Apologies that you're seeing this problem.
Regards
Alan