You might have a duplicate content issue. You will want to check if you have the proper 301 redirect and a canonical command in the head of your code. If you don't have this set properly then the search engines will see the www and non-www versions of your site as duplicate. Also remember that the search engines also by default place this at the end of the url /
Here are two links that can help if this is the issue.
http://www.webconfs.com/how-to-redirect-a-webpage.php/
http://www.mattcutts.com/blog/rel-canonical-html-head/
Hope this helps. Good Luck