Are you using the 0.15 coils in wattage mode? If so - The 0.15 coils are intended for Ni mode and should never be used in wattage mode as it is highly dangerous....
1) You cannot use nickel on wattage mode for two reasons. First, you run a risk of vaping some very objectionable stuff if you overheat nickel. Temp. control solves that issue, but in voltage control it could theoretically get hot enough to behave inappropriately. Second, nickel does not behave like Kanthal. The resistance rises dramatically when heated, so while you have a room temperature .15 coil, when heated the resistance is significantly higher. You'll get far less vapor than a higher resistance kanthal coil. So you run more risk, and get less reward. Nickel is not a good idea used in this way.
2) In temperature control mode, nickel works very differently. That rising resistance is perfect, since it makes the temperature of the coil easy to measure. A properly-functioning mod will take it up to the set temperature (or the highest temperature for the power setting) and pulse power to hold it there. That gives you consistent vapor output.