You reduce the sauce on the stove and it thickens. Wait a while longer and it thickens even more. I found the same principle in my writing. Time spent editing, re-writing, with more input from beta readers, etc. and the plot thickened.
Now, I have reached the point where I am ready to take my manuscript “off the heat” and send it out to a few select literary agents. Has anything changed? Yes, plenty – and especially the main plot. It is far better and brings together the key elements of my story in a more powerful and complete way.