Adsense Revenue Plateau

I've been working really hard, especially in the last 8 months.

I'm trying to find an explanation as to why in the last 4 months my Average Daily Adsense revenue has hit a plateau. In the same period of time, my pages are enjoying more than double the unique visitors and basically the same rate of bounce, duration, etc...

I know this is an awful general scenario, just wondering if anyone has ideas of why this might be. I've optimized my ads as much as I can without compromising the integrity of my site. (I don't want it to look like its only reason for being is Adsense.)

One explanation is that the content bids have gone down in price. But even this doesn't make much sense, because my clicks are about the same each day.

Another possible explanation would be that my new traffic is less willing to click on Adsense ads. But the traffic is coming from the same places, mostly search. (my rankings have gone up across the board, thus more traffic.)

Without me having to go into ultra specifics, does anyone have any general ideas as to why this might be?

Thanks in advance.