There are many correct answers to this question. It is difficult to confidently say what the issue is without seeing the player in action. It could be something that could be resolved by practice. One reason could be height.
The player could be too short or too tall to succeed in the jump shot effectively. Another reason is that a player could have an issue with jumping and releasing the ball at the same time. Jumping and releasing must be done at the same time, and for some people, it is difficult to do those two things at the same time.