you need fuel, airflow into (regulated by the reeds) and out of the engine, and spark at the correct time.
check reeds for cracks/broken petals
pull flywheel cover to check for woodruff key slots alinged between flywheel and crank end, and the key is still present
check for clogged intake from filter to carb or clogged pipe (mouse nests are common)
pull intake boot off carb and pipe from cylinder (1 at a time) to see if it will start quickly, do not run it long like that, it's just for diagnostic purposes.
does your carb overflow fuel when sitting ?
if so it may flood the bottom end of the engine preventing fuel/air flow thru and up into the cylinder
report your findings