There’s a scene in “T-Rex” — a documentary that follows the middleweight phenom Claressa Shields from her hometown of Flint, Mich., to the 2012 Olympic Games in London and back again — that perfectly illustrates the biggest problem faced by women’s boxing. Shields is sitting with two Team U.S.A. representatives and her coach, Jason Crutchfield, in a nondescript lobby after glory and fanfare have receded into a post-Olympic haze. She has returned home after four decisive victories, having won the first American gold medal in women’s boxing. She’s 17. The reps from Team U.S.A. are discussing her sponsorship opportunities.