A lot of trial and error goes into getting a build that works well usually. For the 5 dollar cost it isn't worth the headache to me. Too many variables at play.
Could be too much cotton, too little cotton, too high resistance, too many wraps, wraps touching, not enough power, wicks not fully saturated, the list goes on....it all comes down to learning from the builds that don't work and remembering what you did on the ones that work well so you can do it again.
I never had the patience or the skill to improve on the factory coil. YMMV.Cheers.