File-log plugin to record log 4xx responses

How to configure file-log plugin to log 4xx responses against services, Looking for information on enable detailed file logging of response upon enabling file-log for a service Response with HTTP 400 , 200 and 500 are getting printed but 401 response are not getting recorded , I am using kong 0.9.9 version

Any guidance appreciated

Regards
Irfan Ameen