Are you hiring sales reps or engineers? Why are you asking for an engineer to sell you on their past project(s)?
Interviews by their nature are about judgment, and knowing you're being judged by a complete stranger who holds the power over this potential path of your future is enough to get anybody flustered, especially a brainiac that just wants to work in relative peace behind their desk.
I don't ask them to sell me, I ask them to dive into the technical issues they faced and how they solved them. It tells me a lot more about how they will handle similar issues in the future than scribbling DFS on a white-board would.
I don't understand what you're calibrating for...