I often send out requests to people asking them to provide a quote or let me interview them for an article. Oftentimes, I don't know the person at all; I've just read something they've written or learned about a project they've worked on and would like to either include a direct quote or talk to them. I'm kind of all over the map on whether I get any kind of response at all, let alone whether the other party agrees. I haven't found any kind of technique that increases buy-in.
With my latest one, I needed to talk to doctors, so I wrote to several whom I know personally. Only one bothered to write back and decline. The rest didn't even do that much. It doesn't bother me so much when I don't know the person, but these were people I knew in real life that I've worked with before, so I found the lack of response rather bothersome. Is that normal for this kind of letter, even if you know the person you're talking to? I'm basically offering them free PR, so it's hard for me to understand why they wouldn't reply at all.