Tappan ignition problem

When I switched my Tappan Gas Range (Model No. 30-3991)from LP over to natural gas (orfices changed), some of the burners fail to ignite by spark ignition even though gas appears to be streaming right through the spark! It worked perfectly for over three years on LP. A service man came and was baffled. He left without solving the problem. I have tried cleaning everything, esp. the holes in the burner top where the gas comes through. The spark ignition works fine but the burners do not light unless I fan my hand (move air) over the burner and even that takes a while sometimes. Other times, I give up and grap a match. I am extremely handy. What do I need to do?
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