It's important to properly "
prime" your coils before use, and to "
break in" the coils correctly. The following video is for a different tank, but the principles shown are relevant to all tanks.
All coils have recommended wattage range. Although this recommendation is not followed by all vapers, if you are having issues like you are, I would follow those recommendations.
Coils will not last indefinitely. In fact, they may only have a useful life expectancy of a week or so. E-liquids which are very dark in color or have a lot of sweetener in them will reduce the life expectancy of the coil. Some coils have trouble vaporizing thicker e-liquids (typically those having a high Vg ratio).
Some "brands" are notorious for having a short coil life when compared to other brands. The SMOK coils seem to be notorious for having a short coil life, sometimes being a dud right out of the package.