Electric Ignitors on Cooktop Not Sparking

[SIZE=3]Two of my 4 burners will not spark. They can be lit with a match – so the gas is flowing correctly. I replaced one ceramic ignitor electrode because it was cracked. I replaced the spark module. The replacements made no difference. When I turn the knob of the either of the working burners, sparks are generated for all of the burners. The reverse is not true. What could be the problem – there’s not much under the cooktop?[/SIZE]