You may be an excellent shooter in practice, in the pre game warm-up, or at halftime, but shoot a low percentage in games. Or, you may be able to score consistently when open, but not when closely guarded.
Shooting requires single-mindedness, the ability to exclude everything from one's consciousness, except the immediate task at hand: putting the ball in the basket.
Shooting requires self-assurance. This capacity is sustained and nurtured by the knowledge that one is doing the "right things."
Doing the "right things" is taking the good shot and knowing that the execution of every shot is consistent with its technically-sound movement pattern. An example of taking a good perimeter shot is shooting only when teammates are in position for an offensive rebound.
Shooting requires a high degree of flow. Consequently, when shooting, you should keep your muscles relaxed and your joints, which connect your major limbs, such as the hand to the wrist, loose and relaxed.
The ability to relax is closely related to having a feeling of confidence. The inability to relax when shooting is often the result of stress which is induced by the belief that you are incapable of doing the "right things" related to shooting. The stress which comes from not having confidence makes it difficult to concentrate on executing a shot.
When shooting, you may also be unable to relax, if, when you are shooting, you are thinking about whether or not you will make the shot. To help you relax, focus only on doing the "right things" related to shooting, and not on whether you will score.