In general, when the vapor volume is reduced below a satisfactory level this usually means that your cartridge needs to be replaced. There is a chance that your battery may need to be recharged as well although most switches will blink when the battery needs a recharge.
To be sure, you can remove the cartridge and look at it. Over time you will develop a knack for what the "membrane" inside the cartridge looks like when it needs to be replaced. Usually it will appear extra white in color and dry looking. Also, the fibers from the membrane will protrude from the cartridge when it is dry.