Which brand and type of battery are you using? If easier, you can send us a picture.
Please try the common trouble shooting steps below:
1) For testing purpose, please remove the pocket clip if it is installed.
2) Unscrew the head from the body tube. Unscrew the tail cap from the body tube.
3) Use a piece of clean cloth to wipe the threads on both ends of the light tube.
4) First Install the tail cap and make sure it is fully tightened to the body tube.
5) Install the battery with the button top towards the front of the light.
6) Install the head unit and fully tighten it to the body tube. Most issues are caused by looseness in either head or tail cap.
7) Press and hold down the switch button while putting a hand close to the front of the light
8) If you see the light output when the button is held down, you can loose the pressure and click/tap the button again to test different modes.