When exactly did you transplant them? Transplanting should be done prior to bloom ideally, and after about 3 or 4 weeks of bloom you're not really going to get much new root growth if you transplant.
A lot of things can influence root growth. Roots wont want to grow if it is too dry, too wet or compact medium with limited oxygen\air exchange, if they don't have the nutrients to or pH isn't right (nutrients not available) or the plant is more concerned with producing seed (late bloom).
Otherwise if the plant is getting root bound, when I transplant I simply untwist the bottom roots however I can.