The key to making money with AdSense is targeted traffic. The more targeted your traffic is, the higher your CTR will tend to be. While you seem to be getting your traffic from the right source, organic search listings, the keywords you are targeting may not be the most commercially viable keywords.
You need to consider the level of commercial intent behind the user’s choice of search terms. A higher level of commercial intent will result in more clicks on advertisements.
Another thing to look at is your content. Are you writing a copy that reassures a purchasing decision, or does it create confusion and anxiety about making a purchase? I like to think of my content as something akin to a description of a menu item at my favorite restaurant. Does your content give a mouthwatering description of a wondrous experience that awaits them when they make a purchase from your sponsoring advertisers?