Payment Terms Not Updating

We have instances where we need to change the supplier on an active PO.  The new supplier has different payment terms than the original supplier assigned on the PO.  When the PO Header is revised/saved, the new payment terms are not updating on the PO Header and thus not updating the active PO Detail lines.  We have been unable to establish a pattern that explains why some POs will update to the new payment terms successfully while other POs do not.  The POs we are having trouble with are across multiple branch plants and are basic expense POs.  They are