Beth,
Make sure there isn't anything in the PurchaseOrderNo$ or PurchaseOrderRequiredDate$ fields, That would stop the PO from being created.
When I run into something like this, I typically enter the exact same order/line manually then go into DFDM and look there to see what is different between the one I entered and the imported one. In your case I would start with and focus on the line detail. but it could potentially be something in the header record.