No Power to the Motor Ports of Motor Shield

Hi everyone!

I am working on an obstacle avoiding robot using an Arduino Uno R3 board, an Adafruit Motor Shield (Version 2.3), the Pirate 4WD Mobile Platform, and an HC-SR04 Ultrasonic Sensor.

I have been unable to get the boards to run Adafruit's example motor test sketch, and have tested every variable I can think of. I am getting voltage into the motor shield from an external battery pack with 5 AA's, and have confirmed this by using a volt meter placed on the screws to the power inputs on the shield. The shield is connected to the Uno, and this is confirmed through powering it with the jumper on the motor shield. The Uno is functional, because I have been able to run the blinking LED sketch with no issues. All 4 motors are working, because they run when connected directly to the power supply. I'm sure the issue is due to some oversight on my part caused by my inexperience (this is my first Arduino project!).

I have determined (probably falsely) that my problem is coming from no voltage being transferred from the battery input to the motor outputs on the motor shield. Any ideas as to why I am having this problem?