spark, fuel & air and it all needs to happen at the proper time (ignition timing). You are missing one or more of the three. Replacing plugs does nothing if there is no spark from them. New plugs does nothing if there is no fuel to ignite in the cylinders. If ignition timing is incorrect having fuel and spark means nothing. And finally, if the starting procedure is wrong that needs to be corrected. Are you holding the key IN while cranking? That primes the engine with fuel.