Claudio,
Thanks for your reply.
This issue is resolved 
Configuration was absolutely fine as mentioned above. The only problem was with the authorization of RFC user I used in my configuration!
I was not aware of what authorizations have been assigned to this user in SAP HR system. I thought it has all the authorizations as I have in GRC system itself for this user. Actually, I had not got this user created in SAP HR system, therefore did not consider this in my analysis. I was thinking that it was configuration problem in GRC system.
So some times we miss such small things and have to suffer a lot!
Now I have given SAP_ALL in SAP HR system to this RFC user. It is fetching all managers properly.
Hope this helps others!
Regards,
Faisal