Need help with "Z" holding zero

I was doing a vcarve operation today and was having issues with Z. It seems that it was not holding the correct z height. I originally set it with a tool touch off then stopped the program and manually moved the z lower and set it. restarted the program and the same thing happened. Was air cutting over the material.
Any ideas?
I am going to try a different drawing and see if it happens again as well.
Thanks much!