MRP - PO schedule and creation of Purchase Req.

Hi,

We are running MRP for raw materials with MRP type PD and with coverage profiles. When MRP plans material it is postponing existing purchase orders to future date and creating purchase requsitions and asking to schedule on current date. Existing purchase orders are getting exception message 15 Postpone process. While purchase requistions created by system is having message as 30, plan process according to schedule and it is asking to create convert those Pur Req to PO.

Why system doesnt consider existing PO schedule qty as per schedule and creates Pur req into future dates? Is there settings needs to be mainatined for MRP type firming type? Right now MRP type PD doesn't have any firming type settings.

Thanks in advance.

Regards,

Nilesh