Maybe it was just a fluke. Maybe the muscle memory is not there. An accidental execution on purpose? No matter what you have done, the athlete just seems to elude the duplication of the task. It was a perfect setup, or they just seem to have consistent bad days.
You have pushed. Ran them until you were tired. Tried the skill another way. You have even illustrated it yourself so they can see what it would look like, but it is impossible now for them to achieve. You need to look deeper. We all have good days and bad days. We cannot control the weather, nor can we control someone else. This is a decision that only they can make. In order to prevent injury we force them to go FULL speed. When an athlete goes ALL out, they get much more return on their investment. Why is it that people get hurt, majority of the time when they are idle.
Why is it that in life we say I'm busy so it keeps me out of trouble? Why is it when we coach we say full bore? In life, if we are not fully committed to the cause, we will not get our greatest good from the efforts of partial investment. Don't go through life aimlessly wandering to your destinations or goals. Be on point, go full speed and receive the maximum potential of your efforts. In other words, not half hearted!