If there is no obvious physical fault with the button (e.g. no audible "click" when you press it), it is possible that this is just a configuration issue.
To test if this could be the case import the attached profile
into the Swiftpoint X1 Control Panel, then set it as active on your mouse.
The option to import profiles can be found in the main menu of expert mode as "Import Mappings from a File".
It can then be set as active via the Active Profile drop-down menu at the bottom of Expert Modes window.
The "Button Test" profile is configured to display the name of any button on the Z's OLED screen while it is held down, and light up the RGB logo on Tracer. So once you have it loaded onto your mouse press a few different buttons one by one to make sure the profile is working correctly. Then check how the button you are having problems with responds.
If the OLED/RGB are lighting up consistently while the button is held down there may just be a problem with its configuration in your profiles. Or perhaps the application you are using does not recognize the key you have it set to.
However if the OLED/RGB does not light up and remain on while holding the button. There could be a physical issue with the internal switch, in which case if your mouse is still under warranty please submit a ticket
with the purchase details for your mouse.
If your mouse is no longer under warranty there are still some options to address problems like this. The articles below include details on hardware repairs for the different buttons on the Z (help desk account required).