Make sure your site redirects to either
www.yoursite.com or yoursite.com. If a user tries to login at
www.yoursite.com and then gets redirected to yoursite.com, then they will not be logged in. Instead, set it up (can do in htaccess file, google it) so if you want to use yoursite.com, if they put
www.yoursite.com in their browser, then they get redirected to yoursite.com (or vice-versa).