Tax becomes zero after the sales quotation is converted into a sales order

Hi,

We create sales quotations after that it will be converted into a sales order. We have created 2 taxes in the system. When the quotation is entered tax classification code is selected in the line level of the quotation. Tax is calculated based on the tax classification code selected but when the quotation is converted into a sales order the tax classification code is disappears automatically because of this tax is not in order & in AR Invoice ( the order is interfaced into AR ) . What is the reason for this? Version is 12.1

Thank you