Grab a can of carb cleaner & a pair of red in line spark testers.
Fit the spark testers then go mow.
When you shut down the engine & it won't start again , crank the engine and look at the testers.
If they are still flashin bright red then the ignition is not you problem.
Now give the carm a shor shot of carb cleaner.
If the engine fires then you have a carb / fuel delivery problem
No joy the pull a plug and give the cylinder a SHORT shot of cleaner the refit the plug and crank again.
do both sides , one at a time.
If it fires now but not when sprayed down the carb, you have a valve problem.