The possible reasons to check if reductions did not reduce the scheduled duration of all internally-processed activities in your project after overall network scheduling are if the maximum reduction level is less than 100 or if the appropriate reduction type is set in the parameters for network scheduling. Reductions are functions that allow you to reduce the scheduled duration of activities and networks by changing their work or capacity requirements based on various strategies and criteria. Reductions can help you optimize your project schedule and meet deadlines or milestones. The possible reasons to check if reductions did not reduce the scheduled duration of all internally-processed activities in your project after overall network scheduling are:
If the maximum reduction level is less than 100: The maximum reduction level is a parameter that defines how much an activity or a network can be reduced as a percentage of its original duration. The maximum reduction level can be specified in Customizing for Project System under Structures Operative Structures Networks Define Reduction Strategies

To allocate overhead rates to a WBS element, you must assign a costing sheet to the WBS element. A costing sheet is a tool that allows you to define and calculate overhead costs based on various criteria, such as plant, company code, material origin, or order type. You can assign a costing sheet to a WBS element in the Control Data tab of the WBS element master data. Reference: https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/4dd8cb7b1c484b4b93af84d00f60fdb8/df72bb53707db44ce10000000a174cb4.html

To avoid unnecessary stock cost, you can assign a sales order to a project with appropriate hierarchy levels. This means that you can create a project structure that matches the multilevel production environment with bill of material and routing. You can assign a sales order item to a WBS element that represents the finished product and assign production orders to WBS elements that represent the semi-finished products. This way, you can use project stock to manage the materials required for the project and avoid creating inventory for intermediate products. Reference: https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/4dd8cb7b1c484b4b93af84d00f60fdb8/79ceb65334e6b54ce10000000a174cb4.html?&version=1909.002