Amendment PO message type not triggered

Hello Experts,

     I have an issue during the amendment PO ( In case of change PO ) .In my VOFM routine the sy-subrc is becoming zero but the standard FM SD_COND_ACCESS is returning the sy-subrc as 4.Can you please let me know if there are configurations missed or where exactly i can find the issue is ?

Thanks,

Vijay Simha CR