Stepper motor troubleshooting with Pololu DRV8825 Driver
I think you're driving the stepper motor too fast.

With a 1000uSec high and a 1000uSec low, you're producing a 500 Hz square wave. As such, you're trying to step 500 time a second.

Try slowing things down to maybe 50 steps/second (make your low duration 20 mS, or 20,000 uSec).