For those of us with fine hair, keratin results are usually permanent after the first or second BKT,
though I can't speak for your particular brand. Wash with salt and a regular sulfate shampoo and see if that helps. But, again, let it be known that the presence of the keratin in your hair is not what keeps it straight, but it keeps it smoother and silkier. The finer your hair, the easier the curl relaxation becomes permanent. Meaning--if you have fine hair, your curl relaxation will likely be permanent after the first or second application. Coarse hair will take longer.
I wish I could get that info stickied. My naturally 3c/4a baby fine hair is barely a 2 today...and I haven't done a full-head BKT in well over a year. I feel your pain...crown volume is a daily battle for me. I actually like it when my roots start coming in curly because it gives me root volume. That would have to be my only complaint about my BKT results--it robs my hair of root volume and body.
In the meantime, try some of the root lift powders that you just sprinkle on your roots and they give the roots more body for more volume. And, if you ever have the keratin done again, make sure not to apply the solution all the way to the roots--leave some natural curl there for volume. HTH!