As a plot moves forward, your characters will gather events, relationships, and memories that will transition from the novel’s present to the novel’s past. Make them matter by making them dynamic. If an event or relationship doesn’t progress from what has already been established, you are not using it to its full potential. For example, you present your reader with a contentious relationship between your protagonist and her main competitor on the track team. They make snappy remarks at one another and always vent their aggression on the track. But if their relationship doesn’t progress from this dynamic (by either getting better or worse), this story element will plateau. It becomes something in your character’s past that drags them down.
By having events and relationships change and evolve and grow in importance over the course of the story, you give each story element a trajectory in the plot. Give things a sense of future direction so that they don’t stagnate. In the track rival example, above, I’d find a way to work this relationship into the plot so we know that this dynamic is going to matter in the future of the story. To use a cliché example, maybe I’d work in an upcoming competition to really put the pressure on their bond. This way, the girls aren’t just snarking at one another in limbo, the relationship is also in forward motion toward something more climactic than we’re seeing in the novel’s present.
Think of every important story element as a line that’s climbing toward your climax. Any plot developments or relationships that plateau (especially in the middle of the novel) are shortchanging the future of your story by staying in lockstep with the past. Why is this such a bad thing? The reader is already familiar with what you’ve established. Without a sense that these elements have a future and are going somewhere, a reader’s investment wanes. Remember, a rising line trending toward the climax, with all elements growing, changing, and weaving together.