Two suggestions. First check that the direction signal is actually connected to the driver, there isn't a bad contact or a wire dropped off. Second, check the polarity of the step pulses in configuration, it may be that it is changing at the wrong point in time relative to the step pulses. You could also try flipping the polarity of the step pulses. I'm sure this will be an easy fix once you find the cause, even with the fault it would sell on ebay.