This is a summary of what Google posted on their AdSense Blog.
First things first: eCPM is the metric of the expected revenue from every 1,000 page impressions. Of course, the higher the number, the better. If you run AdSense across multiple sites and industries, you will see this number vary widely. I always recommend spending your time doing SEO on the pages with strong eCPM.
Okay, now let’s get into the fluctuating revenue numbers …
1) Look at the page impressions. Simple enough, but sometimes overlooked. Your revenue could be bouncing because your traffic levels are bouncing. Check your analytics for clues as to why your traffic is bouncing.
2) Check your pages for “unpaid” ads (PSAs). If these are showing up, that could be the reason for your page view decline. If you have quite a few PSAs showing, one of the best ways to fix it is with a statement in your robots.txt file. Simply add this to the top:
By giving the media bot full access to your site, the PSAs often disappear within the hour. This tip doesn’t come from Google, but from our testing labs.
3) You should be using Google Webmaster Central in order to diagnose potential issues with how your site is being crawled.
4) Check the cache of the home page to ensure your site is being crawled regularly.
5) A CTR drop could indicate an issue with site structure, or with repeat visitors who are “tuning out” the ads. You may wish to use Google’s Website Optimizer to shift the AdSense block to test for staleness of your page design. You can check out Google’s Best Practices Tips.
6) Setup Custom Channels so you know exactly where the drop off is occurring so it can be addressed properly.