Problem interview

I am trying to contact potential customers that have signed up for our beta to see if they will take part in a 'problem interview'. I want to find out if I understand the problems my customers face and see if my value proposition is valid. I have tried many variations on the email text below but so far no takers. I have tried this in plain text and html formatted via mailchimp but not a single response. Can anyone help point out what I am doing wrong:

Hi there, (or Hi Joe if I can work out their name from their email address)

Here at [my company] we are busy building our platform for our beta release, but before we put the finishing touches to the design we really want to chat to as many potential users as possible to make sure we build the system YOU want.

Can I give you a call/skype/google hangouts/other to chat about the problems you face in [the relevant industry]? I promise not to try and sell you anything and I promise to delete your phone number/skype contact after the call. If you name the date and time I will try and make it happen.

Kind regards

In my opinion, your entire approach is killing your opportunity. You're email can be re-written as:

Now that we've nearly finished, we'd like to ask you some questions to
help us finish our product.

A different approach would be:

Hi Joe - thanks for signing up to find out more about [product name].
I'd love to hear about the experience and challenges you are facing in
[arena that product fits in] so that we can do our best to make a
product you'll love. etc etc.

You will most likely need to bribe your users to participate. Offer a gift card, free subscription, or something else, that they will find valuable. Otherwise, in my limited experience, you will be hard pressed to find people who will be willing to do an interview.

You don't need to have a lot of interviews as long as your sample is good. The better you know your target population size and composition, the easier it will be to construct a relevant sample. Draw a random sample compose it from a combination of random samples from minority groups within this population (stratified sample).

Alternatively, you can opt for a web survey. People tend to be more responsive if it's just a few clicks online instead of a Skype call with a stranger. On average I get 10-15% percent response rate for a short web survey (2-3 min). This may be good enough for an first stage. Adding a small incentive to the web survey would increase your response rates even more, but may skew the results a little. You can also include a questions if they would be okay to be contacted to clarify some of their answers and ask what incentive they would be interested in.

I would also recommend Steve Portigal's book "Interviewing Users" (published by O'Reilly). If you never done user research before it will be a great introduction to the subject and all the potential issues involved (including legal ones ;-).

Good luck with your research!