Here is a simple drill to train your jump serve.
Draw a line about two thirds of the way back in the court and use that as your service line.
Practice jump serving from this reduced distance. You should find that you can hit it harder, with more angles.
Work on the short cross court serve and the hard line serve. Keep the ball low across the net.
Gradually move the service line back until you are back to the baseline, but try to keep serving with the same speed and angle.
This drill is designed to help you to develop a wider variety of service, and to push the boundaries of speed and angle that you are jump serving at the moment.
This jump serving drill is also great for receivers. If you have more players around, put a receiving team at the other end, and play a side out drill.