I don't know your story because I don't come around often anymore, so forgive me if I've missed something obvious, but tighter curls require more frequent initial applications. I used to have a long, detailed thread here about my BKT journey. I chronicled how I had to do it twice in a row before I relaxed my 3c/4a curls down to where I wanted them. I did my first app, then I did another 2 weeks later, then another a month later. At that time, I was about a 2b, so I waited another month and did it again, at which point the curl was too loose for my taste. I have lost actual count, but that may have been the last full-head app I did, which was back at the end of Jan, I believe.
The looser your curls are when you apply the BKT, the looser the end result will be, which is why each successive application seems to "work better" and "last longer." So, if you are a 4b and you want to be a 3b, I would expect to do 3 or 4 full-head applications in rapid succession--one right after the other. There's no need to wait beyond the initial washout period because it's not like a chemical relaxer. I would do them once a week until my curl pattern was where I wanted it to be, then I would just do maintenance/root touchups or whatever your particular hair type dictates.
My hair has done nothing but grow grow grow. It is now beyond my bra-strap, nearing my waist, and I haven't had it cut or trimmed since Nov. 09. No split ends in sight, either.
My BKT has been life-changing for me. I hope you can get it to work for you as well.