Sand - 2:03 am on Sep 13, 2012 (gmt 0)
You can't say that nobody is seeing gains. I do SEO for a pretty big site (100k+ organic visits a day), and visits have grown considerably over the last six months with only a very minor decline in conversion rate.
Here is my observation based on experience:
As you grow (in terms of visits) sometimes it's your head terms that grow, but most often it's long-tail. As that long-tail growth happens, your landing pages often become less effective because visitors don't see that the page matches their intent quite as quickly as they do when they arrive from a head term search.
Landing pages often need to be adjusted to convert the long-tail in order to keep the conversion rates as steady as possible.
I'm not saying that's the case with you, only that it is often the case.