PI message mapping - header level data will not repeat in line item level

Hey, PI/XI mapping guru,

     I mapped a field in PO header level (source occurence 1:1) to a field in PO line item level (result occurence 0:unbound).

     The header data only show once in first line item in the result and it will not repeat in the following line items.

     What should I do in the message mapping, so the data can repeat itself in all line item level (0:unbound)?

     Thank you in advance!