1. Did you enable those separate features?
2. Did you create an Item Promotion record where THIS PROMOTION CAN BE AUTOMATICALLY APPLIED is checked, COUPON CODE TYPE is Multiple Uses (I assume it's not one-time promo), and AUDIENCE subtab is Everyone (for all customers)?
3. When you're creating an SO, the Automatically Apply Promotions checkbox under the Promotions subtab must be automatically checked and must trigger the automatic adding of the discount regardless of the customer after selecting the item.