I would first review your application as if you were part of an admissions committee. If you analyze closely enough, you could probably find potential areas that you could have improved. Or during interviews, you probably remember several moments where you could have done differently.
With these things in mind, I would first ask openly, "In your opinion, what would you recommend for me to do differently or to improve?" Listen to what they have to say. The first thing you need to figure out is whether the potential problems lie on your interviews or applications or both. And then go from there.