Losing steps on z axis

Hi, on my design it’s sometimes completing and I notice it hasn’;t cut all the way through on later stages of the design. If I start it again, it does cut through, so I don’t think the router or bit has moved, it seems maybe I am losing steps on the z axis?

If there is vertical “give” on your Z or waste board the first pass may meet more vertical resistance (pushed away) which is gone for the 2nd time?

If so, the reason can be flex in Z-assembly, rotational flex of the X-axis or wasteboard bowing during carve.
I would also check pulley/pinion set screws and screw ball bearings for play.