Drop down menu on upper left of BOM tab... If you change the BOM to Parts Only then you will get the total quantity for that part A (but the sub assemblies are hidden). When the display is on Indented the parts will be separately listed under each sub assy. With Top Level the sub assy parts won't be shown at all.
I assume maybe you are exporting the BOM? So if you need it all, but in your desired format, you might have to combine Top Level and Parts Only.
We use an Excel file extracted from EPDM BOM. In this Excel file I wrote an VBA program that analyze data and give the right number of parts and subassemblies. This method has also drawbacks because on every change on BOM we must run program again. There is no automatic update of the Excel document.