For the grill to light you must have an ignition spark and the spark must be in the gas jet from the burner.Can you hear or sea the spark in the grill ? If so the electrode may need repositioning. If not there is probably a fault with the ignition system. On some cookers the spark occurs on the rings and grill at the same time and if the igniter on one of the rings is dirty or wet it will short out the spark and prevent the grill from lighting.
Jeff
October 2008