Need help with % probe

[m_c]

Given an Arduino Uno is being used, I'd guess it's most likely GRBL that's being used.

First step would be to use a multimeter and check if the Arduino pin is seeing a change in voltage. Given the description, I'd guess the internal pull-up resistor for the pin should be enabled i.e. getting a bit under 5V, or 3.3V if it's a low voltage board, with nothing connected, then when the touch probe wire is connected to ground, it should drop to 0V (it's got to be as near zero as possible).