@Tam is correct. The Kanger mini boxes have a spring-loaded pin, but it doesn't take much for them to get gummed-up from leaking juice or dirt. Once they are "sticky", they won't spring back into proper position to make contact with the tank, especially if the tank's center pin is short. Remove the battery to prevent shorts, and then lift the center pin using tweezers or gently "pry" it up using a small screwdriver wedged into it from the side. It only takes a tiny bit to make contact again, so don't try to pull it all the way out