sure your robots.txt file is being used correctly, as it could be denying the engines' spiders entry. Also, try using Google Webmaster Tools to check for any red flags, and run a link analysis tool
like Xenu to see if there's a problem on with in- or outbound links.
One reader's suggestion was to "Check the code for broken HTML elements that might accidentally mask indexable content," as there may just be a problem with the way the site's information is being displayed. "Make sure you're not buying or selling links," said another forum member.