Arduino Car Motor Troubles

As an independent study class in school, I am building a robotic car that is controlled remotely by an XBox 360 controller. I have a USB Host shield and an Arduino Motor Shield R3 stacked on an Arduino R3 board. This is all powered by 4 AA batteries.

I have found that my code and the motors work, but for some reason, when I try to send a signal through Channel A, the motor only emits a buzzing sound, but when I send the signal through Channel B, the motor moves. After further testing, Channel B sends around 5V through to the motor, but Channel A only sends about 0.2V to the motor, and I am positive that both motors work correctly and that the code is sending the correct signals to each channel.

I have tried everything from using only one channel to cutting the VIN and VIN Connector pins on the bottom of the motor shield and even to powering both boards individually. Despite all of this, Channel A still does not receive as much power as it should be getting to make to motor move.

I have run out of ideas as for fixing this. What exactly is wrong with my setup? Is there the possibility that the motor shield itself is broken?

Thanks!