Newbie - Z-axis slipping

I'm brand new, self taught on my CNC router/engraver. I am
currently using a single drill point function where I am drilling 30-
90 holes per part, at a precise depth of 0.043. I am losing depth
starting at about hole 25, and losing up to .030 worth of depth over
the course of the remaining holes. Am I expecting an unreasonable
level of precision, or are there some explanations for why a Z-axis
would lose depth? Thanks in advance!