If you’re having difficulty placing the ball consistently and accurately on your service release, you may be holding the ball too tightly in your fingers. Doing so creates stiffness in the hand and arm that can hamper the placement of your toss.
To remedy the problem, imagine that you’re lifting an egg instead of a tennis ball as you start your delivery. If you squeeze too hard, you will end up with egg on your face. The ball should rest on the tips of your fingers as you lift it gently upward. When your hand reaches about head level, all you have to do is allow the ball to leave your fingertips naturally.
With practice, your release will be more consistent and accurate.