Sales and Use tax on Purchase orders and vouchers

I am new to PeopleSoft 9.2. I am using a third party integration for tax. I followed the directions in the online help to set up the tax vendor. I have order management working, but have been unable to call my tax engine for Purchase orders. Would anyone be able to assist? I have set up the supplier to access the third party tax integration. I have also set up the Purchase orders to calculate tax through our tax engine. The business unit is set up to get tax from our tax engine. What am I missing?