Despite new releases, “How to Train Your Dragon” dominated the box office for a second weekend with $37 million. Pixar’s “Elio” had a disappointing opening, earning only $21 million, while “28 Years Later” debuted strongly with $30 million, marking director Danny Boyle’s best opening. “Lilo & Stitch” continued its success, and A24’s “Materialists” held well in its second week.