Stepper motor stalls at low frequencies. Why?
It's not unusual to use a motor rated at (say) 0.9 ohms and 3A (2.7V) on a 24V supply using a "chopper" driver (the driver monitors the current thorough the coil and goes into a PWM mode when the ~3A current is exceeded, so the motor does not burn out). 

The advantage of this is that the 3A current (and commensurate rotor torque) is reached much faster with the higher voltage. The current, starting from t=0 and i=0, is: 

i(t) = \$\frac{V}{R}(1-e^{-\frac{t}{\tau}})\$, where \$\tau = \frac{R}{L}\$

Unfortunately, your motor seems to be rated at 12V, even so there is some latitude for increasing the drive voltage. You do need a "chopper" type driver (or a high power-wasting series resistor) to keep the motor coils from overheating when they are not being driven at high speed.