They have a safety device - in case the gas supply is interrupted, and then comes on again - a flame failure thermostat - only when it's hot enough is the gas allowed to flow.
Yours is defective in some way.
Peccavi
April 2009
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix gas cookers.