With form_validation class, always jump to index function!

[eluser]Unknown[/eluser]
Hi!I am a dummy to web programming.
In the following controller test.php, I have a function called register which loads a register_view page. After submiting username, password, email, the form is posted to test/create_user.
However,when I'm going to debug this fragment, I find that with form_validation class, test/create_user always leads me to the index of test. Although there are echo upon success of failure of form_validation, there's never message print.

Code:

function register()
    {
        $this->load->view('register_view');
    }
    
    function create_user()
    {
        $this->load->library('form_validation');
    

        $this->form_validation->set_rules('password', 'Password', 'required|trim|xss_clean');
        $this->form_validation->set_rules('email', 'Email', 'required|trim|xss_clean');
        $this->form_validation->set_rules('username', 'Your full name', 'required|trim|xss_clean');
        $this->form_validation->set_rules('repass', 'Re-entered password', 'required|trim|matches[password]');
    

        if($this->form_validation->run()==FALSE)
           echo "oops.";

        else
           echo "form is good";
    }//end of create_user

To be precise, I took fragment out of controller login.php, and tested it in test.php. I made it months ago. It worked well with form_validation at that time. But this time, teammate gave me her final version of web design, good luck ends!
I'm very confused now. Never met situation like this.