Hey Samuel,
The problem could be in fieldcatelog . Please check the field in caps or small, If it is small letter then change it to caps .
perform fieldcat using '1' '1' 'matnr' 'IT_OUTPUT' 20 'STO PO No' 'C810' . " won't work
perform fieldcat using '1' '1' 'MATNR' 'IT_OUTPUT' 20 'STO PO No' 'C810' . " will work
Hope it will help you .
Regards,
Amaresh