When story structure is weak I ask the writer to do two tasks. One is to encapsulate, in 25 words or less, the gist of the story.
Here are some samples:
The Wizard Of Oz: A teenage girl is transported to a mysterious place, and must find out how to get home before the wicked witch kills her.
The Hunger Games: A poor young woman volunteers to replace her sister in a game where contestants must battle to the death to entertain the society’s rulers.
Jaws: The sheriff of a summer tourist town joins with two colleagues to hunt a giant shark that is attacking swimmers.
In each of my examples, I mentioned the protagonist, gave a hint of the inciting incident, the goal, and the source of conflict.
The Wizard Of Oz: teenage girl, transported, get home, enemy
The Hunger Games: young girl, volunteers, survive, contestants
Jaws: sheriff, shark eating tourists, kill shark, shark itself
The first step in structuring a story would be to try to give this briefest of summaries. Try that for your story.
The next step is to expand this by creating a point form ABCD for your story. The letters stand for
A. Ordinary world
B. Inciting incident
C. Protagonist's goal
D. Obstacles that impede protagonist achieving his goal.
Here's how it works for The Wizard Of Oz.
A. Ordinary world: Dorothy unhappy, feeling unloved in Kansas
B. Inciting incident: Tornado moves her to Oz (accidentally killing a witch)
C. Dorothy's goal: Get back home
The obstacles are the source of conflict. Note that, in literature, conflict is not fighting. The sources of conflict are the obstacles that stand in the way of the protagonist achieving his or her goal. Also notice in the The Wizard of Oz how the obstacles get worse and worse. The second last is close to physical death, and the last seems total failure: she's stuck there for good. (Remember, her goal was to get home.)
Those who know The Hero's Journey might add that during the process of overcoming the obstacles Dorothy:
If you are writing a short story instead of a novel, your structure won't be as broad. But you would still benefit from doing an ABCD for it. You might not have time for picking up allies. The mentor might just be the mind of the protagonist himself when he gets a good idea. Seeing a glimpse of the protagonist in the ordinary world, before the inciting incident, might be a very brief glimpse, perhaps hinted to by a memory if you want to start with the inciting incident in paragraph one.
The Wizard of Oz follows classic story-telling format. So do Hunger Games, Jaws, and most police procedurals, courtroom dramas, romantic comedies, and fantasy and sci-fi adventures. This format, which I mentioned above, is called The Hero's Journey. My ABCD is a short version. Try it first to see if you have a viable story structure.