You can try cleaning the battery spring contacts. The battery contacts might have some oxidation that is interrupting the connection between the battery and the light. This is exacerbated by the vibration between the battery and the contact. We recommend cleaning and applying vaseline to all the battery contacts -- a tiny bit works wonders and should last for many months.
There can also be oxidation happening beneath the button and this causes some buttons to get harder to press over time -- this could be happening to your lights. If cleaning the springs doesn't improve the situation, please contact us for repair or replacement under our lifetime warranty.