“Relentless” (** out of four) was an earnest but empty melodrama about an American mother (Lauren Shaw) who searches for her daughter who was kidnapped by sex traffickers in Central America and turns to a local bounty hunter (David Castro) for help. Well-made and well-intentioned but covers awfully familiar ground about the sex-trafficking industry and never becomes fully gripping. For a more searing and powerful look at the same subject, watch last year’s “Trafficked” instead.