Check that the center core of the plug wire is jammed goodly on the pin inside the coil and thepin inside the plug cap. If you are not sure if it is deep enough, take a match stick as a depth gauge, place it in the well for the coil and mark on the stick where the edge of the well is and transfer that depth mark to the plug wire and stuff it in to that depth. Do same on the cap end.
If you have a new cap with the wire and it is a resistor cap make sure you have a non-resistor plug, you can run one or the other but not both (can run neither if you have points ignition).