link from sales order to delivery not complete

2007A

We have a few cases where the link from a sales order to delivery is not complete. For example, we have a sales order with 2 lines on it. We copy to delivery and add. Remarks field has the comment "based on sales order ...." However if you try to get to base document the icon is not available. In looking at the base document, base reference, base line fields on the items they are blank.

We also have instances of this where there were 20 lines on a sales order, we copy to delivery and 19 lines are linked but one is not.

Any ideas as to why this might be happening?