Many players serve with a stiff arm, which undermines their potential to hit their serve with power. A stiff arm cannot act as a whip to transfer energy out through the racquet, which translates to more speed.
There are usually a couple of culprits that I’ve found that cause this problem. Number one, the toss is too high, causing the racquet to pause or stiffen up as you wait for the ball to drop. The key is to lower your toss to a point that forces you to have a continuous serving motion. Tossing just to the peak of your reach, actually makes you speed up the racquet arm and at the same time keep the racquet arm loosey-goosey, which will give you more racquet speed at contact.