If you work hard, but you aren't getting as much out of your practice as you think you should then there may be several different explanations. You might be kidding yourself about how hard you are actually working, or you might be being impatient, or you might be doing it wrong. In that last category, how you practice is more important than just the hours you spend.
Humans are great at finding slightly easier ways to achieve any task they have to do frequently. This incremental change is great, we get a little bit better at it for a long while. But then something else happens. We get a little bit lazier at it too. Start a minute or two later, finish earlier, don't try too many new things.
What the piano teacher, gym instructor, and most education professionals share is a knowledge of this fact. That's why the best teachers try to vary methods and content to keep work fresh.
But it isn't just about variety, it is about a particular kind of practical ability: that of self-reflection. An over-arching skill, no matter what your chosen goal, is to be able to consider recent results and work out how you can improve. Most of us aren't very good at this - think of proofing your own work - and need the help of others to properly critique our work. But even with help, the best situation is to push yourself to find fault with your own work / effort / practice.
Because when you self-identify a problem, you are far more likely to fix it. When someone else points one out, it is so much easier to decide that they have it wrong, and the room for an excuse widens.
Inspired by this article.