If you've got source code for popomta.p this is where you need to look. The way you've got it written, the session trigger is not firing. You might try a database trigger, but I doubt if this will help. By the way the way PO maintenance is written pod_qty_ord is not update until unit of measure is, so you will need to know the frame name also.
Also if you've got a question about MFG/PRO post there, someone may have already done this.