There are two common reasons that discount codes may not be working.
1) You haven't made it far enough throught the checkout to see the applied discount code. When clicking through a discount link, such as those that are provided in emailed promotions, the disocunt will be visible on the final checkout page, not the cart page.
2) Unfortunately, discount codes cannot be applied to subscription orders. Because the item is already discount on the cart/checkout level, no other discounts are able to be auto or manually applied to these orders.