PIR is not consuming by sales order in strategy 40 with backward consumption

Hi Guru,

I am using planning strategy 40 and I have given requirement by MD61 as given below,

January=100 qty.  Feb= 200 qty,  March= 250 qty , April=300 qty , May=350 qty,

after MRP run all PIR get converted into Planned order becuase stock is zero, when I create a sales order havin 80 qty with request delevery date of January month, My PIR will get reduced and became 20 qty because of sales order is 80 qty,

again when i create a sales order with qty 20 of Feb month then My PIR  gets reduced of Feb month instead of jan month, I have set backward consumption mode in Material master with 200 days backward consumption.

Why unconsumed PIR of Jan has not consumed by sales order of Feb month,

Kindly help me in this regads,

Regards,

Zarun