On mine it's the front sparker that's stopped working. I've pulled the burner out, cleaned the top, closed the gap, opened the gap, etc. All to no avail.
In the end, I found a replacement burner on eBay for $15 and bought it. The model numbers seemed to match - but in the end it was the wrong part.
So i don't know the ultimate fix. But that sparker looks like it's hard to replace as there's a small snap ring holding it in there.
If you find the answer, please share!