We just got a used 1983 “King” model Blaze King last year and the thermostat is the only real issue we've had with it. I took the back cover off and the flap doesn't move at all when I turn the thermostat as if it's not catching. Does anyone know how to fix this? For now were just using a little hook to keep the flap just barely cracked open which seems to work fine, but I'd still like to fix it. Any advice would be appreciated.