You'll need to find out how the UC plugin handles probing, but a quick google search hasn't thrown up much details.

As the motion controller generates the motion (steps/pulses), it also has to handle the probing, so even though the probe input shows as working in Mach, Mach is not directly responsible for handling probing. Mach simply sends a probing move to the plugin/controller, and the plugin either returns the point the probe activates, or that the probing move failed.

Hopefully somebody with experience of the UC controllers will be along to answer the question, as it's not a controller I have any experience with. Is there any option within the UC plugin configuration screen for probing?