Back to Back Order Changes and Notification Issue

 Hi, I have created a Sales Order(Back to Back) for ATO Item and after successfully Booking and Requisition Import, I have created Purchase Order against the same. But when I have done the amendments/changes in the Sales Order in terms of reducing the quantity, the Purchasing user is not getting automatic notifications as per the standard functionality. Rather I have to go to the tools and manually send him notification regarding the Sales Order changes. Please clarify on the issue as how to get the notification automatically for the Buyer whenever there is a change in Sales Order.