Form validation callback function doesn't work

PHP Code:
public function unique_app_scanner_name($str, $id){
      $this->load->library('form_validation');
      $uniqueNameCheck = $this->db->query('SELECT * FROM `app_scanners` WHERE `Username` = "'. $str .'" AND `id` NOT IN ('. $id .')');

      if($uniqueNameCheck->num_rows() > 0){
        $this->form_validation->set_message('unique_app_scanner_name', $this->lang->line('in_use'));
        return FALSE;
      }else{
        return TRUE;
      }
  }
  public function edit_app_scanner_validate($id){
    if($this->is_logged_in()){
      if(!empty($id)){
        $appScannerCheck = $this->db->query('SELECT * FROM `app_scanners` WHERE `id` = "'.$id.'" AND `UserId` = "'.$this->get_user_id().'" ');
        if($appScannerCheck->num_rows() > 0){
          $appScannerResult = $appScannerCheck->result_array();

          $this->load->library('form_validation');

          $this->form_validation->set_rules('firstName', $this->lang->line('firstName'), 'required|trim|xss_clean');
          $this->form_validation->set_rules('lastName', $this->lang->line('lastName'), 'required|trim|xss_clean');
          $this->form_validation->set_rules('username', $this->lang->line('username'), 'required|trim|xss_clean|callback_unique_app_scanner_name['.$id.']');
          $this->form_validation->set_rules('password', $this->lang->line('password'), 'trim');
          $this->form_validation->set_rules('cPassword', $this->lang->line('repeat_password'), 'trim|matches[password]');

          if($this->form_validation->run()){

            $this->load->model('model_main');

            $this->model_main->edit_app_scanner($id);

            
            redirect("main/view_event/".$appScannerResult[0]['EventId']."/#tabs-3");

          }else{
            redirect("main/edit_app_scanner/".$id);
          }
        }else{
          redirect("main/view_event/".$appScannerResult[0]['EventId']."/#tabs-3");
        }
      }else{
        redirect(base_url());
      }
    }else{
      redirect(base_url()."site/login");
    }
  }