1) Look at the diagnostic page, and see if the probe LED lights up every time you trip it. It should. And also look at the LED on the UB1, which should also light up (or go out) each time the probe trips. Both of these will tell you if there's an issue with the probe.
2) If you call a G31, and nothing happens, then the issue is not with the probe. (unless you get a "probe active" error message)
How are you calling the G31, and what are you doing with it?
You need to provide more details of exactly what you are doing, and what is happening, step by step.
3) What version of UCCNC are you using?