You should be able to tighten the brake band and get it to draw. If not, I am not sure what is going on other than you may be holding the yarn too long and not drafting fast enough.
I would definitely suggest playing with some "scrap" roving and the brake band though. I've been trying with mine, and am geting close to the perfect twist. As my friend Nell tells me, "it just takes time and experimentation."