Hi,
I suspect that your steppers have microstepping regime of 8 microsteps per full step or more.
When you click the MPG Mach will issue say one or two steps but the steppers will not move until four, five or six steps have accumulated.
This is a common problem with steppers, you might think that if you have 1600 steps per rev if Mach issues one step it will move 1/1600 of a rev,
but not so. This is because the differential torque between micro steps diminishes very rapidly beyond half stepping.
The practical maximum resolution of a two phase stepper is 400 steps/rev (or half stepping) irrespective of the microstepping regime.
Craig