What you have to realise with PPL flying is that it is recreational and many of the participants have different ideas of what recreation is all about. The busy businessman wants to be able to fly, possibly because he thinks it might be another tool in his business inventory. He may not want to go back to school, he may not have time to do lots of prep and above all he may have grandious ideas about his ability. At the other end of the spectrum you will find students who almost over prep by reading so far ahead they confuse more advanced material with what they are supposed to be doing.
Its important that the instructor tells the student what they should do before their next exercise and that they will be expected to start the day on time with the right preparation done. If they don't then you should emphasise why things may not be going accordng to plan and make them aware that its due to their lack of preparation. Sadly, there is not much you can do to improve matters if they do not have the inclination to prepare.
One of the joys of PPL training is that it is not a sausage machine, you get a broad range of interesting people with a wide range of abilities or lack of them. It is a challenge for the instructor to find the best way to provide these customers with a way of fulfilling their requirements as well as teaching them to fly, whilst making allowance for their particular issues, even if that involves pointing out their deficiencies.