You might have the wrong spark plug. Some of the plugs they are putting in from the factory are not running hot enough. Some people are going back to champion RJ19LM tried and true plugs. Just a possibility.
I think I know what the problem is. Your coil/magneto is not putting out enough juice. It should read 2.5 to 5 oms. Set your meter on 20 OMS. You put one end of meter on the coil and one one the end on the metal that snaps onto spark plug. Closer to 2.5 the better. You can go onto you tube and type in "testing coil on lawn mower" and it will show you how to test. I'm betting yours will read around 5.5 or higher.