Sometimes I imagine the plot of a story like a cross stitch, with the front of the stitch being what the reader actually sees; it's the surface of the story. Everything is nice and neat and clear and nearly perfect. But then there's the back of it, where you can see the real work beneath the surface. I like to call that stuff the "undercurrent"--which I guess implies that I also see the plot of a novel like a body of water. There's the surface. And there's the undercurrent.
The undercurrent can be big or small, but it should always be there. In your story, the true plot should be bigger, with more knots and underworkings, than what we see on the surface, otherwise it will feel flat, get boring, and worst of all, leave a reader feeling uninvested in the book.
I see this happen in unpublished stories from time to time. There is no undercurrent. It's a tiny, tiny surface stream of a story. Everything is just as it seems. Everything is straightforward. Everything is visible the first time through. Sometimes with stories like this, readers can't tell exactly what's wrong, but they just know they don't want to read more. They aren't interested. They don't care what happens next.
Every great story has some size of an undercurrent--the backside of the cross stitch, the bottom of the iceberg--or whatever metaphor you prefer. But there is a whole spectrum of sizes.
