Seeed motor shield, stepper erratic

I’m trying to run a stepper motor (Adafruit #324, can’t post link) on the Seeed motor shield 1.0, for now just half a revolution one way and then half a rev in the reverse direction, with a second delay in between. The motion is very erratic and inconsistent, sometimes continuing in the same direction as before, sometimes reversing mid-pulse (even multiple times). I’m using a 12V, 1A wall wart, and I’m pretty positive my motor leads are connected correctly. I have the number of steps per rev at 200, and have been varying the rpm from 60 to 150–outside of that range it seems to just vibrate. Any ideas would be great!