SU53 doesn't capture RFC calls authorisation check
I am calling a BAPI from an external application (creating sales order), and I'm getting a return message "No authorization for maintaining sales documents". This is fine, but when I try to check SU53 for the user involved, I get nothing. Seems like SU53 doesn't capture authorisation checks from RFC calls. How can I find which authorisation object is missing? I don't mean only this particular one (although I could use it too), but generally.