Line is storing in inventory posting table but not appearing in posting form if we select item code as Category

Hi Experts,

I am adding a new main account to prepayment for PO..

So lets go to item group from inventory and ware house management select>setup >inventory>item group >select item group click on posting >purchase order > pre payment > add new line ans select category.

Very first time line is creating and storing in inventory posting table.

Again close and reopen form this time previously added line is not appearing,

again i tried to add same line but not allowing me because we have records with same combination in Inventory posting table.