Plot is a means to show the journey of characters, their desires, and motivations. Backstory drives your characters. If your plot is failing you, it’s probably time to write out some backstory, even it’s only a few paragraphs on each major character. Focus on seminal experiences for each person.
Backstory is the motherload of ore that gives you answers to The Why. Whenever a character takes an action — and that should be often for essential characters, so we see them demonstrate agency — we always ask why they acted that way. Events make up at the heart of plot. But character journeys are the heart of story. We read books for story.
Once you uncover the characters’ desires, your plot will become a servant to your story's people. For example, discovering and recounting the ground zero of a protagonist's abilities — the genesis of a detective's curiosity manifested in journalism, then the way it brushed up against police work, and how that became a new career finding the answers to questions — gives such a story meat on the bone.
Backstory is the way to learn the why about the protagonist's failures, as well as the path to leap across the chasms to demonstrate new abilities. Story determines the choice of plot events, not the other way around.
Work done on characters, especially a hero and a villain, will give an author insights for good judgments. The villain really drives the story's conflicts, so setting them out on paper or on the keyboard can help.
While you may not need to compose complete backstory stretches to appear in your book, you might be moved to do so in selected flashbacks. The flash describes the length of the passage as well as the sudden return to the past. It's 2018. Keep it tight.
Composing backstory is genuine story development. Taking deeper dives into key characters will drive good judgments — from you as the author, as well as the judgments of the characters in the story.