Form validation errors not displaying upon error (EE2)

<?php echo validation_errors(); ?>

<?php echo form_open($action_url); ?>

    <label for="group_name">Name of Group <em class="required">*</em></label>
    <input name="group_name" id="group_name" class="field" maxlength="255" value="<?php echo set_value('group_name', isset($group) ? $group->name : ''); ?>" >
    
    <?php if(isset($group)): ?>
        <input type="hidden" name="group_id" value="<?php echo $group->id; ?>" >
    <?php endif; ?>
    
    <input type="submit" class="submit" value="<?php echo !isset($group) ? 'Add' : 'Update'; ?> this group" /> or <a href="http://<?php">Go back</a>

<?php echo form_close(); ?>

$this->EE->load->library('form_validation');
        
        $rules = array(
            array('field' => 'group_name', 'label' => 'mame of group', 'rules' => 'trim|required|max_length[255]|htmlentities'),
            );
            
        $this->EE->form_validation->set_rules($rules);
        
        if($this->EE->form_validation->run())
        {
            $group_params = array(
                'name' => $this->EE->input->post('group_name')
                );
                
            $this->EE->db->insert('oneliner_groups', $group_params);
            
            $this->EE->session->set_flashdata('message_success', $this->EE->lang->line('group_added'));
            $this->EE->functions->redirect($this->base_url); 
        }
        else
        {
            $this->add_group();
        }