Codeigniter调整大小功能不起作用

The resize function below doesn't work. But if I comment the create thumbnail, the resize line works !!. Why it likes that ?

if ($this->form_validation->run()) {
            $path = 'dist/img/product/';
            $thumbnail_renamed = $this->rename_file($_FILES['thumbnail']['name']);

            ///  Upload image first
            if ($this->upload_file('thumbnail', $path, '', '', array('file_name' => $thumbnail_renamed))) {

                /// Create thumbnail
                $this->_createThumbnail($path . $thumbnail_renamed, '225', '150');

                /// if photo 2 is not null
                if (!empty($_FILES['photo2']['name'])) {

                    // Renamed file firstly
                    $photo2_renamed = $this->rename_file($_FILES['photo2']['name']);
                    // start upload
                    if ($this->upload_file('photo2', $path, '', '', array('file_name' => $photo2_renamed))) {
                        // resize image
                        $this->resize_image($path.$photo2_renamed,'600');

                    } else {
                        $this->session->set_flashdata('message', $this->my_fun->alert('danger', $this->upload->display_errors()));
                    }
                }
            } else {
                $this->session->set_flashdata('message', $this->my_fun->alert('danger', $this->upload->display_errors()));
            }
        } else {
            $this->session->set_flashdata('message', $this->my_fun->alert('danger', validation_errors()));
        }