Ballon relations re picking part or assembly

I have a custom property which I want to use in the ballons, It works except when dealing with a subassembly. I can get the custom property to show correctly in the BOM but when I attempt to pick the subassembly I end up with the custom property for the particular part in the subassembly. Is there any way to select the subassembly so I end up with the correct custom property in the ballon