Want to know why people don't respond?

Simple solution:  ask them.  Now I appreciate that sounds a bit daft - after all, why would someone who didn't respond to your marketing communication tell you why and, uh, respond.  But for an established brand, done in the right way, with a suitable incentive, the learnings of non-responders can be invaluable in helping to refine your communication programs.  For a number of Fortune 500 brands I've worked on we did just that, in one case receiving over 5% response to a short questionnaire resulting in thousands of valuable insights from 'unresponsive' customers.  Of course, in hindsight, we wish we'd done some things differently - like building into the questionnaire enough profile points to be able to attempt a 'mapping' of the results to the main database and support development of predictive modelling.  But bottom line was, we learned a lot - so if at first you don't succeed, just ask!