stepper problem

Hi,

A friend tried to post this question but never made it. Hoping that
someone can point out the most likely cause of this little problem of
steppers moving only one direction.

He has one 2-axis driver box with G201 which only move in the positve
x and y direction. When commanded to move in the negative direction,
the motors only emit a tingling sound, no movement. To eliminate the
possibility of the step&direction signals being the culprit, the
lathe was connected to another driver box (g201's) and movement was
smooth both ways - positive as well as negative.

What is the most likely cause with the problem driver box?

Thanks.

Andrew