Pop Up Drops
It can be demoralizing to hit a drop that sits up perfectly for your opponents. And while people generally blame the height of the ball as being the real culprit, what caused the ball to get too airborne is sometimes misunderstood.
Big Backswing = Big Air
Even if there is no followthrough, a long backswing is a recipe for disaster when hitting a drop. The paddle's momentum gets transferred to the ball making it much more likely that the small arc the players are trying to hit, instead because a rainbow that gets smashed back hard.
While serves, returns, and drives typically use a 'hitting' feeling, the drop is different. If players think of preparing the paddle at impact and then simply 'pushing' the ball forward, they have a much better chance of moderating the speed of the ball. This will make it easier to keep the ball low over the net.
It is much easier to hit a drop well when the player is on balance. Have them prioritize being in a set, stable position when making contact. The activity 'In a cast' is a useful one for focusing on moving rather than hitting.