What you are missing is the basic functionality and capability of Bills of Material.
Subassembly A contains 10 components
...add BOM and 10 components are displayed.
Assembly B is comprised of 2 subassemblies and 8 components
...how can the original BOM (for subassembly A) account for the additional subassembly and components?
Create drawing for Assembly B
...add "indented" BOM.
This type of BOM would display all components and subassemblies including subassembly A and all of its components.