Which brand and type of battery are you using? If easier, you can send us a picture. We also recommend testing 2x CR123A batteries to eliminate that as an issue.
1. Remove the pocket clip if it is installed.
2. Remove the tail cap. Take a look inside the battery chamber to make sure nothing is blocking the contact.
3. Loosen the body tube then fully tighten it
4. Install the battery with the button-top toward the head of the light. Fully tighten the tail cap.
5. Click the power button on the tail cap. Rotate the mode selection ring on the light head.
6. If there is no output, click the power button one more time. Rotate the mode selection ring again.