The use of tax groups in Oracle Project Billing

Hi

Client hasd a tax requirement for one region where they have to utilize 3 separate tax codes for labour transactions. These 3 tax codes are defined ina tax group in AR. The tax group is subsequently assigned to the relevant labour expenditure types. In the iplementation options --> Tax defaults I have defined the sequence of deriving the tax codes for Expenditure Types as 1, Customer Site is 2. When I generate the invoice in Project Billing the tax code defined at expenditure type level is ignored (tax group) but the tax code at customer site level (individual tax code)