$ this-> form_validation-> set_message在codeigniter中不显示

Question

Here is my controller,

class Form extends CI_Controller {

    function index()
    {
        $this->form_validation->set_rules('username', 'Username', 'trim|required|xss_clean');
        $this->form_validation->set_rules('password', 'Password', 'trim|required|xss_clean|callback_check_database');

        if ($this->form_validation->run() == FALSE)
        {
            $this->load->view('myform');
        }
        else
        {
            $this->load->view('formsuccess');
        }
    }

    function check_database($password)
    {
        $this->load->model('testlogin');
        $username=$this->input->post('username');
        $result=$this->testlogin->check($username,$password);

        if($result)
        {   
            $sess_array = array();
            foreach($result as $row)
            {
                $sess_array = array('id' => $row->id,'uname' => $row->uname);
                $this->session->set_userdata('logged_in', $sess_array);
            }
                return true;
        }
        else
        {
            $this->form_validation->set_message('check_database', 'Invalid username or password');
            return false;
        }
    }
}

Everything is work fine except the set_message(); function. What's wrong in my coding. If i provide wrong credentials set_message function is not working.

Can anyone tell me what's the problem.

Thanks in advance.