Darryl. When the cooker initially lights, the small flame should heat up a thermocouple to allow the main gas valve to open up. Sometimes, if a bit of food drops into the burner, it can change the shape of the flame so it isn't hitting the thermocouple so give the burner area a good brush and vacuum. If this doesn't help, it may be possible to gently bend the thermocouple (which looks like a bit of bent metal) so that the flame can properly heat it.
Neil the Turtle, April 2008