It's disappointing when a movie seems poised for greatness, only to stumble at a crucial moment. That's what happens with writer-director Erica Tremblay’s latest film, which centers on an Oklahoma Native American woman (Lily Gladstone) from the Seneca-Cayuga Reservation as she searches for her missing sister (Hauli Gray) at the Grand Nations Powwow in Oklahoma City, bringing along her 13-year-old niece (Isabel Deroy-…