Do you have the potentiometer set close to the "max" current setting?
Your motor is rated for 670 mA per phase, and the driver is rated for max current of 750 mA.
Depending on which version of the circuit board you have, the min/max legend on the board may be incorrect (reversed). According to Easy Driver stepper motor driver, the silk screen min/max legend is reversed on the version 4.3 board.
The motor is not a very powerful one - the holding torque is rated at 950 g-cm (which, if I have calculated correctly, is equivalent to about 13 oz-in).
The easy driver is designed to operate in 1/8 microstepping mode. In that mode, the current through the two phases will likely be different. If you have only wired up one phase, it is possible that the current going through that phase is very small, rather than being close to the maximum available current.I've wired up one phase to see if the motor will hold hard and will not turn