A conflict can be rooted in the main plot, it can be rooted in a relationship between characters, or it can be both. Here are a few strategies for creating a new conflict in a scene.
If a finished draft is too long, or too short, the first step is to create a fresh outline, from scratch. This ensures the outline is accurate, and helps you to review the story as a whole. Ideally you should create multiple outlines at different scales. First, describe each scene in 1-5 sentences. Then, if the story is a novel, describe each chapter in 1-2 sentences. Last, identify the 2-5 key moments in the story.
All stories have three things in common: they begin with a status quo, use a conflict to disrupt the status quo, and end when the character either restores the previous status quo, or creates a new one. Depending on the scope of a story, the entire narrative may take place over a few minutes, or a character’s lifetime; generally larger spans of time lead to longer stories.
One of the most basic forms of engagement is curiosity, what’s going to happen next? Surprises, judiciously used, can keep a story interesting, and prompt audiences to reconsider what they’ve previously read or thought. A surprise can either be a surprise for both audience & character(s), or a surprise for the audience alone.
Once in a while a scene needs something more, a touch of extra emphasis to increase the tension. Usually this means either asking a character to make a sacrifice, or forcing a character to endure and suffer.