I pulled this (well written) answer off Reddit, where I read it a while back...
I know exactly what problem you are having. I have the same unit and I run into this almost every time I refill the tank. The problem is the coil is not making a connection to the base of the unit. There's two solutions to this problem, first one is pushing down and turning the top of the unit left (basically opposite of what you would do if you were taking the top off to refill) as you rotate you will feel a series of grooves. Try stopping in each groove and firing the unit.
Second one is loosening the coil a bit. I have always tightened the coil as much as possible, I realized backing out the coil just a rotation or two would make it closer to the metal base of the tank.
I'm guessing it is flashing five times rapidly? If so, it means resistance too low. Which would mean you don't have the coil screwed on tight enough to the top or it isn't reaching the bottom of the tank solidly enough. That or the coil is slanted and causing the short circuit warning.
The manual here says:
Functions: Short circuit protection:
1, When the short circuit occurs before working, the button light flashes for 5 times and then powers off. 2, When the short circuit occurs while working, the button light flashes for 3 times before short circuit (reset).
Atomizer protection:
eGo AIO has a 15 second run time protection. The light will flash 10 times if the button is pressed for more than 15 second.
Low voltage protection:
When the battery voltage drops lower than 3.3V, the button light flashes for 40 times and the device will power-off. Charge the device to power device back on.
Resistance test:
1, When the resistance at the output end is >5ohm, the light flashes slowly if you press the button. 2, When the resistance at the output end is <0.3ohm, the light flashes quickly for 5 times, suggesting the ohm is too low or potential short circuit.