When going for a stumping, you frequently drop the ball.
You are snatching at the ball rather than watching the ball right into your gloves before taking them to the bails.
It was noted earlier that when you stand up to the wicket, you should always be able to reach the bails from where you take the ball. That applies to the distance you stand back behind the stumps, but the width of the ball determines how far wide of the stumps you must go. Even so, you can help yourself by leaning in towards the stumps as you begin to bring the ball back to the bails. Even when the ball is wide of leg stump, watch its line and trajectory for as long as possible before it is hidden by the batsman’s body. Then step across to the leg side as the ball reappears, watch it into your gloves and, leaning into the stumps, whip the bails off. That is a classical piece of wicket-keeping!
This is an excerpt from Cricket: Steps to Success.