Track Partially Raised PO from Sales Orders

Track Partially Raised PO from Sales Orders

If I have a sales order with 15 items and I converted it to a Purchase Order with 12 Items These items are invoiced and billed, the next time I convert it to a PO i should get 2 items listed but it shows all the items again

Why doesn't Zoho Track the items in Sales Order like it does in Invoice 

There should be an option to do that 

As our team grows, it is becoming an issue for us that wastes a lot of time