Light just purchased. The tail-cap and/or switch seems intermittent. When I click it on , sometimes it works sometimes it doesn’t. While its on, it may turn off by itself. Clicking through the modes, does not always work. When it turns off on its own, tapping on the tail cap seems to solve the problem.
I am all for low prices, but these Convoy Q/C issues are really getting annoying. I would gladly pay an extra $5 per light and not have these random issues.
You can use snap ring pliers (I don’t recommend using any kind of tweezers, they can’t stand the torque you need to really tighten that ring). Just mind that the threading on the retaining ring is reversed, i.e. it screws in counterclockwise.
One other thing to check is good battery contact. I’m not sure about your model but my M3-C has a very long battery tube. I had to stretch the tail spring to make good contact.