Hi Alok,
All the approaches suggested above are correct, as per your note above, Finally You could check whether the document splitting is active for your co.code, if it is active then system will propose the cost center to the other lines. pls check the document simulation with general ledger mode and use expert mode button to see how the splitting rule ...splitting the line items. if it is so then you have to create primary cost element.
Thanks and Regards,
Surendra kumar kotha FICO.