The flat two stage switch doesn`t snap into the tailcap like the long version. The threaded plastic piece and the O ring just below the resistor PCB keep the switch straight and centered so it won`t touch the battery tube or tailcap and come on when the light is not supposed to. Make sure that O ring is on the switch and the threaded platic piece is screwed into the tailcap. The battery negative pushes against the switches spring should be forcing the switch into the tailcap away from the battery tube until the tailcap is twisted tight or the flat rubber is pressed and held for momentary light.