Cutting deeper in areas???

[alboy]

The machine is set to have a safe distance of 10mm above home position so when you home to the workspace the end of the bit will be 10mm from the workpiece surface. I set the machine to go from home position to work position and it seems to be accurate each time. I tested the gap with a piece of 10mm thick aly.
I set up a job to drill 10 6mm holes 0.2mm deep in a line across the x axis over a distance of 600mm. and as the holes are being drilled left to right, the far left hole seems correct but the holes get gradually shallower along the right of the axis. Machine bed spoil board seems nice and flat.

Al