Coil may be weak.
Connections from the plug cap to the coil may be weak.
12 volt supply to the coil may be weak and contact may need to be cleaned.
You may be using the wrong plug.
It may not be properly gapped.
You air filter could be clogged.
You may have over filled the crankcase with oil as excess will get sucked up into the airbox and back thru the engine.