Z axis goes stupid...and I may have found a workaround

[Bob]

It sounds like you are hitting the limit of how far down your Z will travel. That will cause the z stepper to lose track of how far down it actually traveled. Then it won't go down far enough for the following cuts. Guess how I learned that.
To test... Take out your material, router off, no touch plate, and jog your Z down as far as it goes. See if it stops before it touches the depth you want. I'm guessing it will bottom out about .2" too soon.
If this is what happens, look for mechanical causes. Is the router slipping up into its mount? Bit too far into collet? Gantry too high? etc.
Bob

[4DThinker]

You have either hit the bottom of the Z-travel as Bob said, or you may have debris piling up behind the router mount that prevents the bit from reaching the bottom of potential travel. I've also had one instance where something got on the threaded rod that pulls the router left and right stop the router in motion when hit. Of course all the moves in that direction were then off by the distance it couldn't travel when it tried.

[meb]

If you put the shortest bit you will be using in your router then run your Z axis all the way down you might see that your bit can not even reach the full depth of cut you are trying to get. This is my first step in setting the height of my router in the clamp. Loosen the router clamp and lower the router low enough to reach the spoil board then tighten the clamp. Now this shortest bit can cut to full depth and not end up cutting air as you describe.

[Bob]

And, another thought...Some of the Sharks came out with two plastic router clamps. If yours has two clamps, you lost another .75-1." of Z.
Bob