Hello there:- Before you condame the main gas valve ( the combination valve ) You should check out four things. Firstly after you light the pilot remember to turn the dial to ON on the gas valve. Secondly check the thermostate by passing the terminals . Thirdly check the pilot flame is it strong and blue in colour. fourthly check the thermopile (if you don't have a meter to check it ) or change it and keep the old one in case it is still good. It should comes on if not then you can say the main gas valve is defective. Hope this can help. Good Day and God Bless.