You have to "get real" with them and tell them what the performance will cause, in terms of checkrides, jobs, and just outright survival. Some of them can do it, it's just a matter of inspiring and motivating them to do so. For many of them, this is the most they've ever performed in their life, even though it's well within their capability. Be careful and watchout for dogmatic statements and teaching. I've had to demo entire approaches to students to show them the real management of the tasks, when they should be doing things, when they shouldn't, where they should be looking, what to ask for, and so on. I required my CFIIs to do it just for this purpose. Not that we can't teach while the student is flying, but many of them never really get the concepts and are constantly making it harder on themselves, twisting dials and pushing buttons in turns, not requesting time or lower altitudes, asking for position/radar information, being able to task-shed with navs and know what is really important and what is "nice to have", never really getting the concept of using the attitude indicator, and so on. You'll see these situations where you constantly say the "right things", and if quizzed or asked the student will give the "correct answers", but you can watch them do the task in the airplane and see the obvious disconnect, in that it just doesn't happen and can't be applied.

If they aren't grasping these concepts and taking this stuff to heart, you often have to "stop" training until you can get some kind of commitment to improve. Remember that these are usually young people who have never done anything where it was solely their performance that determined a life and death outcome. You are not trying to make a negative implication here, but trying to motivate them to perform to what the situation requires.