This will 99.9% be because you have the wrong Input number in Mach3. You can't trust Mach3 auto detect.

Try changing the Probe input to any of these. 10, 11, 12, 13 or 15. Getting it wrong shouldn't hurt anything, if the input is shared by another switch ie: limit or E-stop all that will happen is when you short the probe the limit will trip.
If you go to Diagnostics tab in mach3 on the right you'll see a bank of Led lights. When you touch the probe the Digitize Led should light up. If it's on already without touching then go back into inputs and change the active Low state.

If any of these input numbers don't work then you either have some wiring issue or the board input is not working. If you have limit switches then you could swap the probe with this input to check your wiring, you'll need to set the probe input to same input number as limit.



Do you have a manual or give more info about the model of machine.