howdy! and welcome!
couple of quick questions - which coil are you using in the subtank? if you are vaping at 15 watts, i am guessing the 1.5ohm coil?
also, what are the nic level and pg/vg ratio on the eliquid you are vaping and how do you have airflow set (wide open? halfway?)
to Esoterica's point, also keep in mind, vaping is NOT smoking - you are going to kind of re-learn how to hit it - but it WILL be worth it...
in regards to putting the coil head in (atomizer), you want to snug it down finger tight - don't try to crank it down too tight, or you can deform the silicone o-ring underneath it and this can lead to leaking issues... also, whenever you put a new coil in, you want to make sure you are priming it well - i typically will put a couple drops of juice in each of the wicking holes on the sides and let em soak in, couple-three drops into cotton from top of coil, then a couple more in side holes... fill tank, reassemble, let it sit for a minute... then i usually do a couple of primer puffs on it (close airflow ring, no power, take a couple of slow drags on it and look for airbubbles coming up from wicking holes inside tank)... generally, to be on safe side, i will give it a couple more minutes to make sure that wicking is good and saturated before firing it up... with a new coil, may want to start a little lower on wattage (like 9-10 watts, take a couple puffs, crank wattage up a couple notches, take a couple puffs, till you get to your 'sweet spot')