convert 20V .4A to 12V .4A
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One thing you'll discover as you grow in your electronics powers is that very few things in life are truly linear. In high school, we teach kids Ohm's law:

$$V=IR$$

And that if you change the voltage while holding the resistance constant, you'll get a linear increase in current. If you actually sit down and do this in a lab, you'll notice that as you increase the voltage, the power being dissipated in the resistor causes it to heat up, and a hot resistor has a different resistance than a cold resistor. 

You may have also noticed that there is often a limit on the current that a power supply can sink (as suggested by @Andy_aka). An Arduino, for example, may only be able to sink/source 40mA of current. So even if you have a 5V pin with a 10 Ohm resistor, you might expect 100mA of current, but would only observe the 40 mA cap your Arduino can supply. In this case, the true voltage that would be coming out of the arduino would be $V = IR$ or, 400 mV.

Finally, if your stepper motor has any mechanical resistance, this will increase the amount of power (P = IV) needed to rotate. Perhaps your stepper motor is trying to move something with a lot of mass?