Losing Shipment Number

We've been having an issue when we remove an order from a shipment and it doesn't generate a new shipment number for the line we removed.  We'll have a sales order with multiple lines all tied to work orders.  The line type of the sales order lines will be W (meaning that they're tied to work orders).  In every case we've seen, the scheduled pick date has been in the past.  All lines have been generated on the same shipment.

When 1 of the work orders has been completed, we want to remove that line from the shipment so that it can be shipped.  After removing the line from the shipment, that line no longer has a shipment number.  It has inventory because the work order has been completed and the inventory has been assigned with a lot number that is tied to the sales order.  In order to get a shipment number, we have to change the scheduled pick date to the current date.