Check the 18650 rechargeable li-ion battery you are using and see if the positive tip is indented. If you are using a different battery that what was supplied with the laser, make sure it does not have a protection circuit otherwise it will be longer. The length of the battery should only be about 65 mm. If the battery is over 2 years old, you may need to replace it. Also check the battery charger if it is still operating properly.
The other possible cause maybe the endcap. Please see below for the troubleshooting procedure.
Troubleshoot the laser: take a small piece of aluminum foil about one half by one and a half inches. Fold it in half so it is now one quarter by one and a half inches long. Take off the end cap, lay the foil across the positive tip of the battery and make sure the foil is touching the threads on the inside of the laser as you put the end cap back on.
Be sure to not get the foil too thick in between the threads of the cap and laser as you screw on the cap as it can jam up the threads and ruin the laser.
The Smart Switch™ should be lit now. Click the end power switch on and off several times. The Smart Switch™ should always stay on now with the aluminum foil in place.
Now use the laser, if it works okay, the problem is with the end cap. Request a new tail cap by emailing email@example.com.
Remember to take the aluminum foil out of the laser when you are done using it, otherwise it will slowly drain the battery over a period of time. Be sure to tilt the laser's back end downward as you take the end cap off so there is no chance of the aluminum foil getting inside the laser past the battery.