Hi,
I suspect that the spindle nose of your new spindle is electrically isolated from the frame or earth of your machine.
The probe circuit is roughly:
1) A current limited voltage on your touch plate (isolated from earth)
2) When the tool touches the plate a current flows from the plate through the tool....through the toolholder....through the motor
armature...through the bearings....to the case of the spindle to earth, the frame of your machine.
3) Mach detects the current flow as a probe contact.
If that circuit is not complete, lets say you have ceramic bearings in your spindle then the current cannot pass through them and Mach
would not detect the fact that the tool has in fact touched the plate.
May I suggest, as an experiment, run a second wire from the toolholder of your spindle to earth. Then try probing again. Until you can
get Mach to recognize when the tool touches the plate then all bets are off.
Craig