Assuming your machine is "square"? If you're not sure check it. If everything is square and your "steps per" are correct on all axis, check all your mechanical connections really well. Something as simple as set screws being loose on a coupler may cause either consistent or inconsistent errors. It can be software related but the majority of lost steps (especially on one axis and when it was previously cutting correctly and you made no software changes) are due to play somewhere.