Hi,you more than likely have a weak ignitor.
Do you have a meter to check for amperage?The ignitor should draw 3 to 3 1/2 amp in order for the valve to open
The ignitor needs to be replaced. The part number is XXXXX and the price is $42.55. You can buy it at www.repairclinic.com.
If my answer helps you, please click on the 'accept' button. That's how we experts are paid for our efforts.