Five things which decrease your shot Efficiency:
Stepping into a shot - one of the most common ways for coaches to teach players how to shoot is through stepping into a shot, which in theory (I think) should increase something (don't really know what...). The reality is that depending on where you are on the floor you're going to use a different footwork as you're preparing for a shot.
Stepping into a shot slows down the time from catching to shooting significantly. A few years ago I performed a mini study on youth players, and tested how much time does it take them on an average to shoot as they were stepping into their shooting position, and the results came out crazy! On an average it took the players 5,3 seconds from catching the ball until the ball left their hands.
Squatting after catching - another big mistake players do is bending their hips after catching the ball, witch as you can imagine will only add extra time before releasing the ball. As I said earlier we have to be set to shoot at the time of catching the ball. Any additional movement we do before initiating ball movement up and after catching the ball decreases your shooting efficiency.
Not fully extending after releasing - also known as a goose neck shot. It is crucial for young players to develop a habit of fully extending both of their arms after releasing the ball. Neurologically, whatever we do after releasing the ball actually affects the release. Also by not fully extending we are adding more variables to the movement by finishing with the hand in a different position every time.
Twisting while extending - pretty common mistake, which mostly happens because of muscular imbalance in our bodies. As far as repeatability, it is really hard to repeat the movement consistency due to its complexity. It is very important to be able to shot as vertically as possible.