I had the same problem (notice the past tense used here) with my son's tfv12. Bought him the new tank and extra coils as a gift. He's been vaping for a couple of years now, so he's not a newbie. I've been vaping for a little over a year, but am a flavor chaser, not a cloud chaser, and thus,do not use these tanks. Juice was pouring out, no matter what he tried. I talked him into leaving me the tank, while he went back to college,to see if I could figure out the problem. I finally, after trying everything I could find online about the problem, thought that the black o-ring on the coil might be the issue. It looked a little small. I grabbed a spare o-ring from a smok tank (not from another coil, the replacement o-ring for the tank itself). I replaced the o-ring on the coil with the wider/thicker (not bigger diameter) one for the tank and have had no leaks! The new o-ring will have to be put on successive coils, and this may not solve everyone's problem, but it worked for us!
This forum won't allow me to post pics, so if someone wants to see them, contact me. I have pics of the coil with out any o-ring, with the new one, with the old o-ring stacked on top of the new one on the counter do you can see the size difference in thickness.