If you got this printing log message :-

No print-relevant changes to document 9912345678 exist

Message no. ME 140

Execute this program RSNAST0F in SA38 to do the printing.

Object key                : Your Purchase Order Number

Output type               : NEU

Transmission medium : 1

Output application      : EF
 

Best regards,
SAP Basis, ABAP Programming and Other IMG Stuff
http://www.sap-img.com