A lot of the time issues involving seemingly empty cartomizers are actually down to the battery. The amount of power a battery can deliver will diminish over time, and a dying battery will not be able to vapourise the liquid contained within the cartridge to the same level it did previously which may cause the cartomizer to seem empty, or taste odd.
Due to minor differences between coils in cartridges, the battery may not be able to fire one cartomizer at all, but can fire another one just fine. It's always best to try the problematic cartomizers on a newer battery first to eliminate the possibility that it's the cartomizers causing the problem.
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.