So one side sparks and lights but keeps sparking, the other side does not spark so does not light.
The one that does not light, you should be able to light with a match or other, above the spark electrodes, is another electrode, which senses when the flame is on, to stop the spark, if they are in correct position, Not been knocked/bent out of position or anything, you have a faulty spark generator.
SD, December 2011
