A lot of the time issues involving seemingly 'blank' cartomizers are down to the battery. Over time the amount of power a battery can deliver will diminish. A dying battery will not be able to vapourize the liquid to the same level it did previously which may cause the cartomizer to seem empty.
It is recommended to change your batteries every 2-3 months so if yours is older than this it may be time to purchase a new one.
I hope this offers an explanation as to why you are experiencing this problem and if you want to have a look at our replacement battery options, please click here.