Not sure,but sounds to me like only the pilot light is lighting and the gas supply valve is not opening to allow the main burner to ignite.
The problem may be a defective igniter/thermocouple or a defective gas supply valve. The igniter/thermocouple is a single unit; once the pilot lights and heats the thermocouple, the gas supply should open igniting the main burner. The igniter/thermocouple is easy to replace and not overly expensive; replacement of the gas valve is a little more work.
If you have a way to apply 12vdc to the gas valve and the valve opens, then the problem is with the igniter/thermocouple. If the valve fails to open, then the gas valve is defective.
Ray
