file to idoc scenario: SNDPOR value is not picking from mapping

Hi

I have multiple file-->po-->idoc scenarios. In the mapping I am populating control record field SNDPOR value correctly. But during runtime it is not getting picked. To make this worked I am populating SNDPOR value in idoc communication channel (override Idoc control headers). Any idea why it is not picking from mapping ? How to solve this problem ?

Thanks

kumar