If you really think it's AdSense that's your sites' problems then the implementation might be a problem, maybe too much above the fold or something along those lines. But then again, that would apply to any advertising, not just AdSense.
Or it might be something totally different. Just to hear you say you think AdSense might be a problem sounds to me like you could be at loose ends at diagnosing what the actual problem might be. Maybe you're a little to close to your sites and should get a different opinion or two.