Drop ship sales order - add quantity or line

Hi

In Drop Ship sales order we have a few orders where the shipped quantity is not equal to received quantity. The reason is that the received quantity had 2 different lot numbers and at the time the under shipment tolerance profile was to high. The first lot got shipped and invoiced and the sales order line went to status closed without shipping or invoicing the other lot number. When adjusting the under shipment tolerance profile this is not a problem anymore.

I need advise to how to invoice the remaining lots since the sales order line is closed and I can not add a new sales order line to the order. The order head is still open.