Vaping causes carbon depositing on the coil so you have to burn it out at least once in two weeks.
To do it, remove the atomizer body and press the button on the battery until the coil burns red. Some recommend to use a gas or turbo lighter for that, but that is mostly done on fresh coils to remove the metallic taste in the vapour, and you should also be careful with open flame as it may melt the silicone isolation ring on the positive post.
Burn the coil several times. Now heat it once more and immediately put in cold water. Gently peel off extra hard scale with a sharp blade. You may wipe the coil with alcohol. Now heat it once again to remove moisture and alcohol, add a drop of e-juice to the coil, reassemble the atomizer and give it a nice long puff. It should feel fresh and delicious, almost like when vaping on a new coil.