If the engine is turning over and you're 100% sure that the plug is good and is putting out enough heat try resetting your needles to the what the manual recommends. Be sure that the engine is getting fuel- check for blockages in the fuel line and debris in the tank.

If it still isn't working then describe exactly what it does when you try to start it. We can't tell you how to fix the problem if we don't know what the problem is.