Ox CNC can't cut small circles

I recently build my first CNC router, the OX kit from SMW3D. So far everything is going well except for the fact that I can't seem to cut circles. As you can see from the picture the circles aren't quite round. I am using HSM inventor with the TinyG post processor so i don't think it is a software problem.

I have cut other parts with rounded corners and the corners seems to be fine. From what I can tell the flat spots are parallel with the x and y axis. Has any one else had this problem?...

Ox CNC can't cut small circles