Perhaps you should post a compacted/zipped db copy for analysis. If you have PO id (master) on main form and subform (child) I cannot envision what the issue is as long as you are committing a main form record before trying to add child records. If that is not the case, then you're still inputting data in the wrong fashion. I can only imagine that is because a) you're trying to insert subform records first, or b) you're starting a main form record first but for some reason, it's not being committed before you start adding po list items in the subform. Simply moving the focus from the main form to the sub should be enough to commit the po record so it's puzzling if that's the scenario.
A db sample would cut to the chase. All that's needed is the 2 forms and whatever tables support them and instructions on how to replicate your issue.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.