The easy answer is that the clear tank will be empty or the oil has settled below the aperture holes and can no longer flow into the coil chamber. It is recommended to use the oil in a 510 cartridge within a month of purchasing to avoid any clogging or leakage. When not in use, store the vape cart in a vertical position with both the mouthpiece cover and bottom silicone cover in place to avoid any evaporation or leakage.