Depends...on how its wired, could be a bad connection. I don't use the bike frame to ground anything, no need to. All my builds have new wires (both Blue and Black) soldered, and heat shrink, don't use any connectors.
You can make a coil voltage tester very cheap.
One lead goes to Blue, the other to Black.. doesn't matter which one goes where. Remove wires from CDI, remove spark plug, spin crankshaft with a drill or ratchet. It won't take much.