Why would an open line item from my Inbound Shipment be missing from Create ASN?

Summary:

On multiple occasions, our receivers have reported not being able to receive all open line items from a Supplier Managed inbound shipment in the Create ASN page. Item setups have been compared and match to working items on the same shipment. The line item missing from current PO Inbound Shipmnent on ASN page did not have the same issue 3 days prior for a different PO#. The item also has not had any changes to its Purchase Agreement or PO scheduled ship date in that timeframe either. What could be the cause of this randomly occurring?

As a workaround, the receiver has performed a regular Receive Expected Shipment transaction on the missing ASN line, but this is not ideal as it then leaves the PO line and Inbound Shipment line open and must be canceled.