Hello Ryan,
Thank you for the response. Unfortunately, I do not have an oscilloscope. I have a digital voltmeter and checked the supply voltage. It seems to be ok. I also did try separate power supply i.e. used 9V batter to feed Arduino board and use 6v batteries for the shield. Another test I did also change from NiMh batter to alkaline batteries (a bit higher voltage). Both of these experiments did not improve the problem.
I do not think I see the “Mx fault” messages in the serial monitor. I have not tried to run the shield without motor connected. That is a good suggestion. The Arduino’s pin 6 and 12, you meant the pin 6 and 12 as labelled on the Arduino board, correct? I visually inspected them and they appear to be ok. I will look around this area for more.
Thank you,
-Pandit