piatkow - 7:24 am on Aug 19, 2010 (gmt 0)
Three possibilities come to mind at once:
1. Poor wording of your ads - the "call to action" just isn't as good as you think it is
2. The keywords generating the most ads are those that are least likely to reach potential customers
3. You are in a sector where visitors to publishers sites have a high degree of "ad blindness"
My first thought is to look very carefully at the wording of your ads
My second thought is to try suspending the most unproductive keywords