session issues on ie

I have code which logs someone in and then allows them to see certain info if they are logged in. I have users who claim it works fin in some of their browsers, firefox, safari, etc and some versions of IE but not all and I'm stumped. Here's the code: //this is from the login routine if ($this->db->getField('user_id')){

                       session_start();

$_SESSION['user']['user_id']=$this->db->getField('user_id'); $_SESSION['user']['email']=$this->db->getField('email');

                       return true;

}

       <?
       //only show if they are logged in
       if (isset($_SESSION['user']['user_id']) && !$_REQUEST['mlsid']){

//show something }

       ?>

People are claiming it tells them they are logged in yet they don't see what is displayed under the code which checks their session. For some reason, sessions don't seem to be there on all browsers yet they claim that cookies and all are on. Any ideas? Thanks!