Using old plug? Hmmmmmm....
Jog my memory here. Did you have the engine apart , Jug off? If so, what did you use for assembly lube?
My thought pattern is maybe something coated the plug, shorting it. Maybe you have HV but not spark across the gap becuase a chemicl is shorting the ceramic. (remember, simple stuff first.....)
Try pulling the plug slightly away from the ground source (engine??) and see if it sparks across the air gap. NOTE, make sure you are isolated from the metal of the bike or you might light up your life ....