Another help could be the outstanding qty of the rows, because in case of previous partial deliveries of the rows you have to mind calculate the quantity to insert in "This Delivery Quantity" field.
Then I've seen that, in case of a previous partial delivery of a row, is not possible to set the row as "Completed" with "This Delivery Quantity" zero.
This could happen if the supplier communucate that the item is out of stock or discontinuated.