parent_request_id is not populating in fnd_concurrent_request table

Hi,
I am using below API to launch concurrent request inside plsql block which is running through concurrent program.

           l_request_id := APPS.FND_REQUEST.SUBMIT_REQUEST ( application => 'application_short_name',
                                                             program     => 'concurrent_program_short_name',
                                                             description => 'program_description',
                                                             start_time  => NULL,
                                                             sub_request => FALSE,
                                                             argument1   => prama1,
                                                             argument2   => param2
                                                             );

both parent and child requests are completing successfully but parent_request_id is showing as -1 for child program in fnd_concurrent_request table.

Please let me know if i am doing something wrong or what to do to populate parent_request_id.

thanks,
Niraj