Hi and welcome Brian! What Will said.
The switch is rather simple in its operation and for it to not be working, it is not making contact when and how it should. This is likely due to foreign, non conductive debris on the contact surface somewhere.
Pop the switch out of the shell and try it in the battery tube by itself. If you hold the PCB by its edges and force it against the spring tension until the forward face of the PCB contacts the rear of thebattery tube, you should have low light level on. The electrocal path is from the spring to the "main body" of the switch. From the main body, the path is into the contact with the inner face of the PCB, through the resistors to the outer ring of the PCB and then into the lip of the battery tube. If you then push the button against the back side of the PCB, you provide a path from spring to main body to button to rear face of PCB. From the rear of the PCB, the path goes through vias in the PCB to the front side outer ring of the PCB and into the battery tube.
I believe it is possible to eventually wear through the the tinned copper on the PCB itself but I have yet to see or hear of this happening. :shrug:
