I have a Chinese 6040 router that increases the z-axis height with every pass. I set it to zero, start the program (Mach 3) and with every pass, it lowers a little less.

I am working on a long carve and put the machine on pause for the night. I went back to the shop this morning to continue and everything started fine. In a few minutes, I noticed that it was cutting in the air. It was not going down to the wood.

Any idea what the problem could be?