Reminds me of Steli's story of how he followed up 48 times (!) with an VC who went dead. Zero, zipzilch, nada. Response to the 48th email? "Thank so much for following up, I've been crazy busy. Can you come by the office tomorrow 1pm?" (rephrased). He invested. Of course, you have to do this the right way, he laid out a pretty good "follow up plan" in this post: http://blog.close.io/follow-up