It is always difficult during a match to determine what may have caused a mistake. However, it is extremely important to identify the cause so that you can make an adjustment and hit the shot better the next time.
Toss the ball a little more forward when deciding to serve and volley. It should feel as if you naturally need to take a few steps into the court after making contact with the ball. Continue in until your opponent is just about to start their forward swing.
Many of us are geared towards hitting the ball through our opponent with power and depth. Using the drop shot in addition to this idea can make the court seem very big and difficult for your opponent to cover.