personally i found the 510 cartos to be really iffy. some ran at 12-15w with no burnt taste, while others i couldn't even come close to that. equal resistance though…
same with the i06. i bought 2 and one can run at much higher wattage than the other, even though they're rated the same.
With cartos, the real variable that's beyond our control is the filler material. That can really affect the wicking ability. With the types that you're buying, though, you should be getting better results.
I'm stumped.