I believe your users are catching on to your Adsense ads and how they are blended into your site. Therefor they are not clicking them as now they know they are ads. Just my guess.
I can tell u the answer. bcoz i faced to the same problem, Adsense ads are mainly choose according to keywords, page content,and channels. in your adsense report (earning report),check channel data and payments for clicks. for some channel,adsense show specific ads for most of the times. some times those ads pay less (around $0.01). Identify lower pay channels and remove it. you can change text/image also. try it and send me the feedback.