Providing electricity to Arduino via VIN

Hey all,
I have weird problem. I'm triggering a stepper motor via DRV8825 driver chip on a breadboard. Everything runs swimmingly, electricity is provided by a 12V, 2.4A DC adapter connected to VIN/GND on the Arduino. If the program is running and I disconnect the USB, everything still runs as planned.

However, if I start up the Arduino without the USB connected, the motor just makes sad little noises as if not getting enough current, maybe. Plugging the USB in For just a moment starts the program and everything works fine.

Is this a known issue? Do I have to tell the Arduino where to get its current?

Thanks for your thoughts,
M

A schematic would be much appreciated even if hand drawn along with the sketch you are using.

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I'm going to guess that somewhere in setup you have a while loop that waits for the serial port.