Check the base of the bulb Newbie, see if there are any small cracks around the metal that screws into the socket. I've had a few that came in from Amazon for my bearded dragon that had a bunch of little hairline cracks. I tested them off the enclosure just to see if they would work and they did the same thing. I ended up just tossing them because of safety concerns and started going to the store and just paying more for them, since I can take them out and inspect before taking them home.