1. Make sure the tactical ring near the end of the light is fully tightened, or it will prevent the tail cap from making contact with the light body.
2. Make sure the head and the cap are both tightened.
Your light might just need a simple reset, try these steps in order:
1. While holding down both the plus (+) and minus (-) buttons, click and hold the power button on the tail cap for five seconds and release. You should see the plus and minus buttons flash blue indicating you have a good battery.
2. While holding down the plus and minus buttons, continuously click and release the power button on the tail cap. Click the button at least six times.
3. While holding only the plus button down, press and hold the power button on the tail cap.