Two things come to mind.....
First, it is easy to get the Step and Direction signals swapped. Double check
that you have Step on pin 3 and Direction on pin 2.
Second, the PMDX-131 is an older board that was designed back when all
Gecko drivers required +5 as the Com signal, and this cannot be changed
easily on the PMDX-131. Unfortunately the popular G203V drivers REQUIRE
ground as common. If you have G203V drivers, you have a problem.