sequential number not showing

The sequential number in my invoice is not displayed.

Do you know what could be the problem?

Woocommerce 1.6.6
Wordpress 3.5.1
WooCommerce Print Invoices & Delivery Notes 2.0.1
WooCommerce Sequential Order Numbers 1.3

http://ww.wp.xz.cn/extend/plugins/woocommerce-delivery-notes/