You probably have experienced failing to reach a specific key, or I should say, "falling short" of what you were trying to achieve on the piano many times. Today I'm going to discuss why it's happening and how to fix it. But let's create a very simple scenario here first. I'm going to name this piano student, Susan.
"Susan's right-hand finger-1 (thumb) is on the middle C. And, her pinky (finger-5) is supposed to reach treble C, which is an octave higher than where her finger-1 is. But her finger-5 always falls on treble B, just one note below where she needs to go. The piece she is playing is a very busy piece with both hands. So, she doesn't have enough time to look at her right-hand finger-5 to make sure that it reaches treble C. She normally knows how wide the interval of 8th (octave) is, without looking at the keys. But somehow her finger-5 still keeps falling on the B."
Most likely, what's happening to Susan is psychological rather than technical. She knows that she doesn't have time to look. That worries her a little bit. She may not be aware of her worry. But she may be subconsciously less confident in this situation because she can't look at her right-hand pinky and treble C. This feeling of reduced confidence can easily make her timider. Her slight worry can hold back her forward energy. Her energy is falling short. Therefore she falls short on what she is trying to achieve.
So, what can you do if you run into this kind of subtle situation? It's hard even to identify that your energy is slightly falling short. But if you keep falling short just right before or right below the goal, most likely it's not because of your technique. It's because of your energy withdrawing. Once you realize that, then, you have to do the opposite. "What do you mean by opposite?" you may ask.
Ok, let's go back to the original scenario with Susan. What you need to do is physically experience reaching the farther key such as the treble D or even E rather than C. Just play and extend from finger-1 on the middle C to finger-5 on the treble D or E. This will make you gain back the confidence. It may sound strange. But we all know that fear disappears when you do a harder thing than what you were trying to do. Also, trying to reach the treble D or E will make you feel more comfortable about making a mistake. It'll make you snap out of perfection. And all the sudden, you become more relaxed about the outcome. Then, you become bolder.
The example I've just created is rather simple. But I'm sure there are many situations where you couldn't figure out why you keep falling short only slightly on the piano. The important thing is to be aware of your subtle energy withdraw. And be bold and go farther than the actual goal. Just like an athlete runner, imagine the goal is a little farther than where the actual goal is so that you don't slow down before the goal.