The facial mask is a one piece applicator so the circuit will close before the mask is applied. This can also give rise to voltage limitations as described below. Low powers are better on the face, combined with longer treatment times and of course a pulsed current iontophoresis machine.
6 volts is normally adequate for the treatment of the face and if you find that the voltage will not pass this level this will be due to low resistance offered by the one piece mask. If there is not enough resistance in the circuit the unit will not be able to deliver enough voltage to maintain current so it remains at the maximum voltage it can steadily supply.
Are you wearing the mask when this occurs? Is the unit remaining at 6 with the mask in place on the face? If not, please fix the mask in place and you may find the additional resistance offered by applying the mask will enable the power to be increased somewhat. Also as skin dries and resistance in the skin increases you may find that voltage levels attainable will alter.