Motors not running is usually a result of not enough power.
Read_Voltage.py example and post the battery voltage. It is located under
If the batteries are full it should be 12V. The motors will be disabled when the drops under 6V. However if it appears to be low you could try a fresh pair of batteries just to make sure.
If that doesn't work also try updating the BrickPi3 firmware. That can be done through the Di_Update GUI. There should be an icon for that on the desktop.