V.C.
It pretty much just either 'will' or it 'won't' grow (long AND/OR thick). Maybe someone else has some suggestions, but as the mother of 2 grown girls (and 2 grown boys with all different kinds of hair), I'm of the opinion that you're either born simply to grow hair or to not!
31 yr old daughter (oldest) had almost no hair til she started school and still doesn't have thick hair. Boys had a little bit, and the older one (almost 29) had light blonde hair as a little boy and still has thin, fine hair (like his dad), although they're both medium brown color, now. 2nd son now has THICK, DARK, slightly wavy hair. Youngest daughter (20) has average thickness/coarseness pretty much like mine (except mine's straight as a board and hers has a slight wave to it).