When I ask a follow-up question, I usually hear nothing anymore. I'm still trying to figure out what works best.
I wouldn't change that if I were you. The reason why they don't reply back is because they are probably thinking screw this guy, there are other people who can whitelist me. I'll just try to trick one of them. If the user is legit, they shouldn't have a problem answering a few questions and removing any doubts that are maybe there.