Check your plug connections and wires. Second, check the magneto connection and the point quality. Cleaning your plugs could also solve this problem, as well as making sure that they are properly gapped. Plug wires could have been damaged and have a short, so check continuity. Improper grounding can also be a culprit.
This sounds like the magneto is no good. Its always good to try a different plug when testing. and the spacing between the magneto and flywheel could be off.
Check the plug wires - if loose or worn, that could prevent sparking. Try new wires (or used ones known to be good) to test for this. Also, make sure the spark plug itself is not fouled, which would also prevent sparking. If the plug is fouled, either scrape the carbon from the electrode, or replace the plug.