Many Students are taught how to hit volleys when they are in the offense, the ball is easy and you have plenty of time to move your feet to the ball. However, in reality, you are in the defense hitting many volleys; these are balls that are wide, low, hard, etc.
 
When you are in the defense, your opponent is attacking you while you are at the net and you do not have plenty of time to move your feet. I teach my students to Reach - Reach - Touch - Catch. Once you rule out that the ball is coming at you, do not wait for your feet to take you to the ball. Instead, imagine you are on the edge of a swimming pool. Then imagine that the ball is going to land in the pool and get wet. So, you start to allow your racket to Reach for the ball, before moving your feet, then lay out (like a tree is falling) and Reach again. Next, instead of trying to swing at the ball, just try to Touch it, with firm wrists, but with only the intention of making contact with the ball to keep it in play. At this point you will be in the middle of falling. Let your feet land to Catch your fall, you will get wet and land in the pool.
 
As your tennis goes to a higher level, this will be a great aid.
 
Good luck on the courts!