Need Help!- Stepper motor error & laser engraver

[LaserLord]

Hi everybody

I'm building a laser engraver. I'm using DM556 drivers and stepper motors by Leadshine. The problem is that whenever I try to cut a circle the result isn't round. On all the right, left, up and bottom sides there always are small straight lines not arcs. It seems like that the stepper motors aren't responding well when they have to do a few steps. I think they skip steps at the abovementioned sides. I tested the controlling system on some machines by other manufactueres and it worked perfectly. This is really wierd!!
Is the problem with the motors and drivers or in the mechanical system? The mechanical system is a X-Y table driven by timing belts.and the load on motors is within the limits of their capacity.

Thanks in advance