Check if the battery has any dent on its positive tip.
If the battery is 2 years and over, it may need to be replaced.
If you are using a new battery, it must not have a protection circuit or it will be longer.
The laser is designed to use an unprotected 18650 li-ion battery, this is 65mm in length.
Check if the battery was inserted correctly. It needs to go in with the negative end first.
If the laser seems underpowered or is flickering, check if the "pin" (safety interlock) on the tail cap is loose. The laser will not work if it is removed. Also check the positive terminal on the battery if it has any dent.
If you have access to a multimeter or a voltmeter, the charger volt reading should be around 4.2 volts and the battery should be around 3.6 volts.
To see if the endcap is causing the problem, please watch the video:http://www.youtube.com/watch?v=6v6jJyZKJiM
And follow these instructions:
1) 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.
2) 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 endcap back on.
3) Be sure to not get the foil 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.
4) The Smart Switch should be lit up 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, it means that there is a problem with the endcap's contact points and will need to be replaced.