I sent an email to around 30 people. I have been following the lean startup advice of getting feedback from users to develop the product. I was about to abandon developing since I couldn't even get them to talk to me for about 20 minutes.
I don't think no replies from 30 people is unexpected - unless you made it clear that you would be contacting them for feedback when they signed up. I'd probably shoot them a personal one line e-mail asking them one more time.