Getting code to execute on system generated transactions

I have written a small piece of code that goes through the line items on a purchase order transaction to set a column field to be the date of the purchase order.  This is to allow us to say on the bill when the order was made.

This code is working well for manually entered transactions (I have the code executing on save record) but it doesn't work for automatically generated purchase orders (e.g. drop ships or memorised invoices).  I have to go into edit mode on the PO and then save again.  I'd like it just to run without me having to do this.