DVD drive stepper motors with polulu a4988 on Arduino
According to Pololu's website, the VMOT pin needs at least 8V to function. Try a 12V switching (important!) wall wart supply that can supply at least 1A instead of your USB supply. Avoid supplies that can deliver currents >3A, as they are overkill for a small stepper motor and can be dangerous if shorted. Be sure to put a large (100uF is good) capacitor across that supply to prevent voltage spikes. To prevent damaging the motor from the higher voltage, follow the information Pololu provides on setting the current limit and set it to about 350mA. If you notice the motor is too hot to touch, try 200mA.