12mg/ml nic & 18mg/ml nic will be very harsh using subohm resistances. 3 ~ 6 mg/ml nic are more common for subohm ranges.
What is the PG/VG ratio?
This. I'm about 99% sure his issue is from the nic. I used to do 12/16mg nic. I finally got myself my first Sub-Ohm when I got Halo's new Reactor. Glad I took the suggestions to cut the nic content down. I dropped to 3 and I can feel it stronger than I ever did with the 12.