Pop the coil: sudden loss of firing. Replace your head or device. Burn the wick: lack of liquid or just lots of use. It starts tasting burnt. Replace your head or device. Clog the wick: lots of use can clog the wick with stuff that doesn't vaporize. You'll either see vapor taper off or taste get bad from all the junk around the wick/coil. Clean it if you can or replace it. Break the device physically: cracks can cause leaks, etc. just replace the device. Basically, when flavor/vapor tapers off, or it tastes burnt, or it leaks: replace the head or the whole device. At the price of these things, I tend to skip even attempting to clean the heads. Just grab a new one and keep going.