PBI Matrix Row Subtotals - Applying subtotal to specific column

I am trying to get the "Subtotal" or total to total on SO vs "Customer PO Line Number".

It was subtotaling under SO as shown below 

Then I added the customer PO line to rows and it moved the subtotal to customer PO line. 

I have spent sometime messing with Row subtotals and flipping switches and changing drop down boxes to include SO. (fyi: drop down box was on All before) Nothing has helped move the subtotal back to SO. How do I get the subtotal to fall under SO again?