Coils can burn out for various reasons. This can be due to the coils not being fully saturated with e-liquid when originally primed, or by using the coil at too high a wattage. Most coils have a wattage range displayed on the side, and it is advised that you stay within this range to prolong the life of the coils. Chain vaping is also another factor that can contribute to this as you are not allowing the coil to re-saturate with e-liquid, therefore vaping on a dry coil.