I get it, OP. For $30+ an hour, plus agency fees, I would also want to feel strongly about the person I'm hiring. That said, people are often very different in person than on paper (for better or for worse!). I'd interview the ones you feel on the fence about, and have very specific questions about the areas where you have concerns.
Unfortunately the bar for nanny quality around here is extremely low because there are so many desperate parents.