problem with the coil.
problem with the spark plug wire.
the spark plug firing voltage is determined by the ignition system it is connected to.
this assumes the spark plug is gapped to the correct setting, per the manufacturers recommendation.
gap doesn't make a significant difference in the firing voltage, but it will affect the timing. smaller gapped plugs fir sooner.....wider gapped plugs fire later. the reason for a manufacturers gap recommendation is for timing.
for the voltage to be lower than normal typically indicates a problem with the ignition system.
the coil generates the spark plug voltage.
the spark plug wire delivers the voltage to the spark plug.
you asked for two.
vehicle electrical system problems can cause the ignition to malfunction. poor ground connections are the most common cause of all vehicular electrical problems.
lack of a proper ground connection can cause a low spark plug voltage.
issues with the wiring between the ignition trigger and coil can cause a loss of spark plug voltage.