The key to unlocking your serve is to keep a light grip. If you are griping your racket too tight, it will create tension in your wrist, resulting in locking up your arm. Your arm needs to be loose and relaxed in order to move your racket smoothly and more quickly. Many players approach the serve as a punch, hit or push. The serve needs to be a throwing motion or a “release”. You need to feel the racket accelerate as it moves toward the ball. This is accomplished by holding your racket with a light grip tension.
Here’s a drill you can try to get more racket head speed, feel more rhythm and become more fluid with your serve.
In your service stance, hold your racket as if you’re about to serve. Position your hand low on the racket handle so that your pinky is slightly or even completely off the end of the racket. Doing this will prevent your hand from gripping your racket to tightly. Now without a ball, holding your racket this way, start going through your service motion as if you are actually serving. When you finish your swing, don’t stop your racket from moving and transition from the end of your serve motion to the beginning of your next serve and repeat. Continue repeating this several times in a row without stopping, transitioning from the end of your service motion to the beginning of your next service motion. The key while doing this drill is to relax your grip and to keep your swing continuous.
Next, try adding a ball to the mix. Take a practice swing without tossing the ball, then without stopping your motion as before, toss the ball up to serve. Keep repeating this until you get the same feel on your practice swing as you do on your serve.
Try this serving drill and let me know if it helps!