MRP Backward scheduling

Hi Friends

I am working with my client, here MRP they have Backward scheduling, My problem here is.Backward scheduling is not working for My FG material.

FG has one SFG component and this SFG has one ROH component..

If i created PIR for this FG on this day 16.03.09.

ROH Purc.Requ is creating as per my 2 days GR proceTime and 1 day Dly time.

My SFG is starts on 19.03.09. having one day inhouse prd time,

whereas my FG has one day In-house prd time., But it starts on today itself, 16.03.09.Actually it has to start on 20.03.09.

Pl help in me where the config missing