motivation

How does one motivate someone else? When I normally think about this question, my response is to show the person their need to be involved in the deal and/or show them what it would be like without it. But most students in dropback scenarios have no motivation to learn, even though they've already failed the class and know that they will not move on unless they pass. These students have come face to face with the consequences of their actions (or non-action), but they still have nothing driving them to put the effort in that is necessary to complete the tasks at hand. I would love to be able to figure out how to motivate these students. They simply seem not to care and don't want to put the extra effort in. It might be that I intimidate, but if they cared enough, wouldn't they fight past that? It might be my sarcastic humor, but, again, if they cared enough, wouldn't that be something to look over? I am stuck with my personality traits that push away the "at-risk" and my drive to see students succeed.