Clean out the gas tube. Remove the igniter & clean the electrode off, sand it with very fine sand paper or steel wool. I've heard it might also help to take a wire, poke it through the hole in the gas tube that the igniter goes in (while you have it out) & scratch the inside of the tube to make sure you've got some bare metal that will help for sparking between the electrode & tube.
Perhaps I should mention, that while I've done this on a couple of performers that I've owned & I've even gotten a tank recertified and filled, I've never actually tried out the ignition system. I just use a chimeny (or now two for double stacking). But, it does make sense & I've don the same thing on my oven's igniter electrode & it worked like a charm & saved me from buying a replacement part.