Flexible Sales Accounting using both the Business Unit and Cost Object

We use flexible Sales Accounting.

When I set it up using the Object Account Number taking the Busines Unit from the F4102, when I run the Sales Update, the Account posting is correct.

When I add a 2nd line to take the Cost Object from the F4801 and run the Sales Update the flexibles Sales rule is ignored and the Account posting is incorrect, but the cost object is entered correctly.What do I need to do to set this up to ensure both the Account postings is correct and the Cost Object is entered?