Next time you give it a try if it doesn't start right away check for spark. Basic mechanics dictates that engines need 3 things to run, air, fuel and fire. Air is almost never the problem, fuel also means is the mixture correct but fire is about the easiest thing to check to make sure the spark is bright blue and not orange or none existent. If all 3 things check out Ok then it is likely a mechanical problem such as timing either with the valves or ignition. Good luck!