Problems story writers face

I've been a fan of Antonya Nelson's stories for some time - though have read them primarily in the NYer and not in volumes. Not to knock the short-story form, but I wonder why she hasn't tried her hand at a longer form (maybe she has and I've missed it); even Alice the Great (Munro) gave it a shot. In any case, Nelson is a true devotee of the short form and her stories always bring something to the table. She's observant, very witty, and she has tremendous insight into the lives and minds of young women, usually very intelligent, often Midwestern, in difficult and often unconventional famly situations. Current NYer story is case in point, from the title, which will make you think twice (and maybe turn to Google if you don't get the reference), Primum nole nocera (my spelling is probably off, sorry) to the puns and misperceptions (girl thinks her mother, a psychotherapist, is asking callers if they are "cell-phoning," when in fact she's asking if they're "self-harming"- ha - though a bit of a stretch). Nelson accurately captures the conflicts that occur in a family where one of the parents is a psychotherapist and can say almost nothing about the work that consumes her life every day - and the patients call all the time, anyway. In this story, a woman shows up at the door while teenage daughter is home alone; woman sneaks her way into the house where she will wait for the mother to come home. Daughter feels increasingly uncomfortable, and eventually threatened, and manages to call her stepfather - quite a bit younger than the mother - who comes home and tries to put things right, without a lot of success. In short, an excellent premise for a story, and Nelson had me with her all the way - but I will raise the quibble that the story, once the highly competent and outspoken mom returns home - just sort of ends without much resolution. Imagine what Joyce Carol Oates would do with this set-up - or maybe don't imagine. Nelson bumps up against the problem many story writers confront all the time: I've created characters and a situation, and now what? Short stories can get away with being just a scene, a perception, a moment in time - but the more they depend on a traditional story arc, the more we expect the arc to complete the design. I felt a little let down by this story, which does not fulfill its, or my, expectations.