AX 4.0 - Delivery date in reports not updating

Greetings!

Within my company we have a report that is used to show all open purchase orders(PO). This report is called the "Open PO report". As far as I can tell this is a custom report and it has some minor coding in it (written by our MS partner). An example is the code below:

display dlvdate AVA_PurchDeliveryDate() 
{;
    return PurchTable_1.DeliveryDate;
}

Now for my question. When a PO is made the delivery date is put into the order. It's quite common for the delivery date to be changed afterwards however, this is not reflected in the report. The report will still show the original delivery date.

What could be causing this? I know some fields don't update until you've executed a certain step within AX; is this the case for this field as well?

Thank you for any/all help you may be able to provide.