Hi Rajesh,
With Shelf Life, this is not recommended, because it makes it more likely that a receipt and a requirement which are far apart will be allocated with each other.
Additionally, please check whether the firmed receipts (and those inside the Planning Time Fence) can actually be allocated to the demands. It may be that the receipts expire before the minimum shelf life of the requirement, making it impossible for the system to allocate them.
Regards,
Tiago