netmeg - 4:57 pm on Mar 3, 2013 (gmt 0)
I don't believe there's a cap; I've more than doubled my earnings every year since 2005 (I realize that this is not everyone's experience, but if you can use one example to possibly prove something, I can use one example to possibly disprove it)
You could be distracting your users with your direct ads. Your users could be moving towards a demographic that doesn't click on ads as much. Your advertiser mix may have changed, or what they bid may have changed. Competition may change (yours and your advertisers') You may be serving more mobile users, and in many niches, they don't click on ads as much. Your traffic may be coming from places that typically get smart priced. You may be running into ad blindness, or more users using ad blockers in their browsers.
Sure, anything's possible, and maybe Google is throttling you. But why would they do that? The more you make, the more they make. And if it got out (and eventually it would get out - an ex employee would blab, or a government investigation would find it, or some other way) it would be horrible PR for Google and make them look stupid, and one thing Google doesn't like is to look stupid.
In short, there are a TON of things I would look at before I'd jump to Google deliberately throttling my earnings.