Arduino Motor Shield LEDs only flash when using bluetooth but not through USB

My problem is that if I use the bluetooth shield to control the motor shield (through an android app) it won't work unless it's also connected to a computer through USB. And when I send the input that would get the motors running, the 4 status LEDs for the motors blink (when not connected through usb) and I hear a sort of click. I have an arduino uno r3.

Thanks for your help