Heater won't stay lit. Its a 75-125,000 BTU propane forced air Reddy Heater, serial number

Question

Its a 75-125,000 BTU propane forced air Reddy Heater, serial number 08/3005. The fan runs and you can hear the igniter firing. As long as you hold in the red gas button the heater will light. Even after 1 minute if you let the button off the heater will go out.