If you are using the kit supplied boot replace it with a better one from auto or motorcycle parts shop. The stock boots are low quality. If that doesn't fix it then get new plug wire with copper core, not the carbon type. If that doesn't fix it try a new plug.
two things to try - in case the spark is just not seen, hold plug in hand and hand on good motor ground and turn motor to see if you feel spark - in case plug wire is bad, take wire off CDI, stick finger into CDI where wire goes, put elbow on motor ground and turn motor to see if you feel spark