piatkow - 11:52 am on Jan 17, 2012 (gmt 0)
I assume that you are referring to your ranking in the SERPS rather than Page Rank which is an absolute score for the page.
There are several possibilities come to mind here:
1. You have optimised for the wrong keywords, these are not terms that people use to find your topic
2. Adsense places some more enticing sites above the organic search results
3. Something about the results makes people think this isn't what they are looking for and they drop down to the sites in 2nd and 3rd place
4. Your site is so specialised that that is all the traffic there is