A big chunk of baked on crud gets on the coil and also blocks the airflow somewhat. This can happen in under 10 ml with VG dominant, or the sweeter flavors. If you use cartomizers that you can see the coil and clean/dry burn it properly, you can get at least 3 times as many uses from them, with them almost working like new each time. I clean mine as soon as I notice I am having to hold the power button on longer or to where the draw starts getting a little tight. That clump of baked on crud will completely bury a coil and it can no longer heat properly since it has to now heat this mass of crud that is also soaked with your juice. The coil is having to heat a wick perhaps 3 times it's normal size, not only inside the coil, but now outside it as well.