Possibly due to the Step pulse being on the wrong side of the slope. Commonlly step pulse is on the rising edge but in some cases drives may be set so on the Falling edge. If this is the case and on wrong side then can lose one step with every direction change.

To check this write program that goes back n forth few hundred times on that axis. Mark the start point and check it goes back to same point after the moves.

If doesn't then change the Step low active in Motor Outputs. This will put the Step pulse on the opposite edge.