There are 5 big mistakes that are made by players during the racquet drop.
One is not keeping the racquet face closed as it drops.
The brain does not think that the ball will be able to be hit if the racquet face is closed, The player tries to correct this problem by moving their wrist and making the racquet face vertical as the racquet drops.
To fix this problem keep the 30 degree bend in the wrist and keep the arm relaxed so gravity makes the racquet drop. If the arm is tense there is a better chance of moving the wrist as the arm muscles are already activated.
Another big mistake players make is not keeping the racquet on the right side of their body.
You should be able to put your back up against a fence and swing at a tennis ball without hitting the fence (at least not until the finish).
By keeping the elbow up through the unit turn the racquet can not go behind the back.
The third big mistake is not getting 12 inches under the ball.
Most players will only get low enough to be level with the ball.
For most people getting under the balls flight path feels unnatural, plus players feel like they will miss the ball completely if they are under it. To help get under the ball make sure the hitting arm is relatively straight
Make sure the knees are bent
The fourth big mistake is dropping the racquet head instead of bending the knees.
While this does put the racquet head under the ball, it will put the racquet out of sync with the body and make the racquet much harder to control.
The fifth big mistake is not stepping correctly into the ball. Players who stay open stanced will tend to not transfer the weight correctly onto the right foot and tend to swing horizontally instead of vertically. They will also tend to hit the ball late and not get far enough out in front of their bodies
Some people will go into a closed stance which will tend to keep them from rotating correctly through the shot and leads to less power. A closed stance will also tend to make players take extra steps which will make them slower on the court.
Other mistakes with the racquet drop include dropping the racquet too fast and starting to uncoil the body too early.