Sometimes we can spend a lot of time working on something without making any kind of progress, and that may be a sign that the time could be spent improving something else. If you think about a second year student playing a simple Bach minuet vs. a concert pianist playing it for an encore, we would realize the new student does not playing it very musically at all. But they reach a point where there is not much more for them to learn from working on it at that time in their development, and the teacher will say good job and put a check mark by it.