I examined the story I wanted to write and could not come up with a subordinate plot. I was not plotting a past versus present story. I was not following a secondary character's trajectory. I was not braiding two or more story threads together.
Agatha Christie mysteries are linear stories.
The Harry Potter books are linear stories.
A linear story starts at point A and winds its way to the end. There can be twists and turns, but you essentially follow the protagonist, perhaps with a few detours to follow secondary characters or the antagonist. There is a central problem with layers of conflict along the way: internal, external, interpersonal, and antagonist. Linear stories are quite satisfying. Your camera stays focused on the main stage. The camera can travel to view secondary characters interacting with each other and the antagonist to create obstacles.
It is far better to have a strong, well-developed linear story than attempting to stuff in a B-story to pad your plot.
A weak middle can be remedied by layering conflict. Story Building Blocks: The Four Layers of Conflict explains how.