The biggest problem that I see with new CFI's is them trying to do touch and goes before the student has grasped ground effect.
Stop trying to get the wheels on the ground every approach. Work on getting the student to fly the low approach getting stabilized in ground effect. Once the student has a solid grasp on ground effect, the plane will land itself. Teaching this way, also instills the idea of, if something doesn't look right, go around instead of making a bad landing and potentially having a big insurance bill to worry about.
Trial and error causes headaches and a sore lower back.
try this method if not already using it, and I bet you will see a change on your students success rate.