A plateau is almost never what it looks like
The flat lesson, repeated across four or five weeks, is the most common moment in long-term private coaching, and the moment most often misread by the coach in the room.
The instinct, when a client stops visibly progressing, is to change the work. New drills, harder material, a different piece, a new programme. The instinct is honest and almost always wrong. Most plateaus are not failures of the work; they are the surface of something else, and changing the surface usually deepens the plateau rather than ending it.
The first job of the coach during a plateau is not to fix it. It is to read it correctly. Three different kinds of plateau show up in private practices, and they need three different responses. Naming which one is in the room is the most useful thing the coach can do in any given lesson.