71% of your tweets get NO reply
From http://blog.sysomos.com/ they took an Exploration of Retweets and Replies and found only 29% of tweets actually get a reply and 92.4% of RT's happen in the first hour.
So study the tweets that DO get a reaction and over time you might find some correlations in content, style, headline or even find its the same people RT'ing you. Understanding the behavior of your audience might help you refine what you share and how you post that content to get a higher response rate.
Check out @BrianSolis post on the Science of Retweets on Twitter for some tips on making your posts more retweetable