This is a tough one to answer, as you are effectively saying there is nothing wrong, except that it won't start!
You need very little to make a motor run. A good spark at approximately the right time, a reasonable gas/air mixture and some compression will always at least make it fire occasionally.
A good spark means it has to be a fat healthy spark. Often, you can see a spark when the plug is in the open air, but it is not powerfull enough to jump the plug gap under compression. Even with a good spark, if the insulator is leaky, the spark current can leak to ground, if that is an easier path than jumping the gap, when under compression.
If you are suspicious, check that the timing is close, firing close to top dead center, then install a new plug, splash a small amount of gas into the intake and see if it will fire.
If you get a short burst, then it quits, you have fuel problems. If it won't fire, you have ignition problems, possibly a leak somewhere else in the ignition wiring.
You can check for this by using a spare plug with at least twice the normal gap. In the open air, you should still get a good spark, if you do not, the plug cap, wire, coil or magneto is leaking.
Try this and let us know what happened.