Muneer,
Is your prebook order confirmed? If the ATP qty is 1000 now, but your prebook order is still confirmed, then it implies that your ATP check is using lead time, and your prebook order lies outside of lead time. Please display OVZ9 for your ATP check, and also CO09 for this particular situation.
Purchase orders and stock are ONLY reserved for specific sales orders in MTO.
Best Regards,
DB49