The engine needs 3 things to run: Air, fuel, and a spark. The other things that can fail (and be easily identified or fixed) besides what you replaced are air filters, something in the electrical system (spark plug wire or other components in the electrical including any safety switches or loose wire connections), or cracked or loose fuel line. Easiest one is to check and clean or replace the air filter. For the fuel, check your fuel lines for cracks or leaks. Even a tiny crack in the fuel line may result in the engine pulling in air rather than fuel which definitely will make starting the engine difficult or impossible, For the electrical, other than checking for loose wires or adjusting any interlock safety switches, I would probably take it in for service. Other things that it could be would include a clogged muffler, a dirty or out of adjustment carburetor (I'd let the repair shop diagnose or repair these items), or very old fuel (IMHO, a low odds cause). Not sure it this helps but at one time or another, air filters, loose wires, sticking safety switch,and fuel lines were all problems I have experienced that I have easily checked and repaired. Good luck.