As someone who ALSO does these interviews, it sounds like you're doing more harm than good. Interviews are never the place for adversarial questioning or psychological games. If you wouldn't feel comfortable writing your questions and their answers verbatim in your interview feedback, you shouldn't ask them.

Getting a useful signal is as much about the interaction as the questions, and if you're acting like a jerk then the most appropriate response for a strong culture fit is for the candidate to not want to work with you. Which is probably not something you're capturing very well.