Bound by a common language and torrid history, 21-year-old Mexican nanny Lucila (Anna Diaz) bristles against her surroundings in Madrid, where she and her younger brother followed their mother from their Mexican hometown. “ Ashes ” shares these broad strokes with the novel on which it’s based — Brenda Navarro’s evocatively titled “Ceniza en la boca,” or “A Mouthful of Ash” — but even those unfamiliar with the book might still be tipped off to the haphazard nature of Diego Luna ’s adaptation. The film plays out like a tale where too much has been relegated to the margins and left between the cuts, where the performances shine but their emotional foundations have been laid in reverse.

First-time filmmakers can usually be forgiven some of these sins, only most viewers might not realize that this is Luna’s fifth go in the director’s chair. The actor has found tremendous mainstream success as a fixture of “Star Wars” spin-off “Andor,” but back on Earth, his talents behind the camera have unfortunate limitations.…