My waterheater won't stay on. I finally got the pilot to stay on after several tries. After the piolot light stayed on I then move the knob to the on position, then I adjusted the thermostat to position 1. The burner came on but went off after 1 minute. What else do I need to do to keep burner on? The model # is GS640YBRT, 38 gal capacity, made on 12-06-2004