If you're not getting the ignitor sparking (you should hear it) when you turn the burner knob on for that burner, it could be that either the ignitor is dirty or caked with some gunk and needs to be cleaned.
The other issue could be that the ignitor is faulty and needs to be replaced. If you want to confirm a faulty ignitor before ordering a new one, you can swap it out with one of the other ignitors on the cook top. Be sure to unplug the cook top from the outlet thou, to prevent getting a shock, as those little sparkers can give you a pretty good jolt!
Just remove the grates and burners to get at the ignitor for cleaning it. To get at the ignitor, youll need to go under the cabinet or counter that the cooktop is installed on in order to remove the ignitor and swap it with one of the other ones. You'll see that these are held in place with a retaining clip you can remove with a flat bladed screwdriver, etc.
If you find that the one that isn't working is faulty, you can order one from your local appliance service store, or get one online. www.searspartsdirect.com
lists parts for almost every appliance there is, just enter in the full model number to get a parts list, etc.
Hope you find this Very Helpful and best regards!