It is very common that beginning, novice and intermediate players don't come to the net fully after hitting their return of serve. This puts them at a significant disadvantage when their opponents are hitting a third shot. Let's look at why it happens and what can be done.
Being up at the net puts pressure on the servers when they are hitting the third shot since the net players are better able to pounce on a high ball. And the closer to the net they are, the scarier they seem. Make sure they understand why coming to the net after the return is important and that they know that hanging make makes them a lot less scary to the other team.
Fear of the Drive
Players will often hang back if they believe they cannot handle the speed of the oncoming ball. Remind them that if they hit their return of serve deep, the ball won't be as challenging since it has farther it has to travel.
Additionally, help your players to develop solid volleys so that they aren't scared of a little speed.
Move Like You Mean It!
Have players return serve and get to the NVZ before the ball gets to the server. See if they can 'beat the catch'.
It is useful to remind them that a higher return will allow them more time to move forward.