The front-center burner on our cooktop won't light. When we turn on the gas, the ignitor sparks maybe once and then stops, while the ignitors on the other five burners continue to spark. Since this was the most commonly used burner I suspect it got gunked up somehow. I've cleaned all the external parts I can see and gently scraped the exposed metal parts of the ignitor with an emery board. Sometimes this helps for a while (or maybe it's just coincidence) but it seems to be getting harder and harder to make this burner light with its own spark. It lights fine with a match, so there is no problem with the gas.