Ms
HR,
I have read your post a few times tonight, and keep asking myself. What is the information that you are after?
It sounds to me that you have a pre-conceived notion of why an individual has left your organisation, and your exit interview, is a tool to re-inforce those notions. You may want to be open minded. I am sure that your using the right questions, most exit interviews are pretty straight forward.
In the organisation that I work with, they have a basic questionaire setup consisting of 5-10 'seed' questions (depending on the persons position). It comes down to the person conducting the interview to extract any further information they feel relevant.