Could be a lot of things.. but everything is probably not as it should be if it's not starting..
What I can think of:
-Leaky crank seals
-Damaged/leaking reed/intake
-Carb issue
-Ignition angle off (flywheel key?)
Is the spark plug wet? Is there a good suction pulse from the carb when kicking it? With some experience you can feel it if you cover the carb with your hand on full throttle while kicking/cranking the engine. It should not blow, only suck, and the pulse should be 'strong enough'. Old fuel can make it hard to start too.
