Suggestions:
Each work order created must be firm planned. When the work order is firm planned, the system considers that the order is to be executed. Based on the BOM definition, the process route is set, the machines and/or equipments used in the Manufacturing process are reserved, the material requirement is computed, for specific items and the Inventory is allocated (reserved) for that work order.
Based on the BOM definition, At the time of workorder firm plan, if the Inventory of any item defined as critical in BOM, for the specified BOM is not available, then the system prevents the firm plan operation. Also it depends on BOM master that the allocation type determines how the Inventory will be allocated at the time of workorder firm plan.