Make sure that you have the same URL in your Site Address and WordPress Address fields in your WordPress settings. This means that you need to choose www or a non-www URL in both fields. You may not use any other combination so be certain that this is the case.
Add this to your wp-config file
For more advanced users you can try to debug your site. Switch the debugging option on through your wp-config file.
define( 'WP_DEBUG', true);