Tax rule is not derived on sales order

Hi,

We have configured a tax rule based on Ship-to tax "classification". The rule has been tested and works fine in AR but somehow it is not picked up on the sales order. On the sales order the default tax is picked, so it is not a matter of getting the tax on SO, but the system does not pick the prioritized rule on the SO. Any idea what are causing this or how we can address it?

Thank you in advance