The issue came down to a conflict. The {{layout handle=" sales_email_order_items" order=$order}} wasn't working because the block it was ...
... from administration) Here is my code in sales.xml <
sales_email_order_shipment_items>
{{layout handle="sales_email_order_shipment_items" shipment=$shipment order=$order}}. and when I create a new shipment, I receive the ...
{{layout handle="sales_email_order_shipment_items" shipment=$shipment order=$order}} {{block type='core/template' area='frontend' ...
... {{layout handle="sales_email_order_shipment_items" shipment=$ shipment order=$order}}