So I'm building a arduino robot. I am using the SN754410 Quad Half H-Bridge to drive the motors, with 3 AA batteries external to the arduino. When I plug in the USB cable and upload the code it runs! But my cable is only 3 feet. I bought a 9V battery adapter and plugged that in instead of the USB and nothing happened, not even slight movement of the motors at start-up. So I built a flashing LED circuit and that works fine with the 9V battery.
Is there something obvious that I'm oblivious to?