Why is my discount / promo code not working?

COUPLE THINGS COULD BE GOING ON.

We try to keep things fun with various promotions and offers, which means you'll likely receive emails or ads that express different offers at different times. First, codes are valid for set periods of time - make sure you understand clearly when certain offers expire. Second, if you're trying to apply codes on top of codes or other promotions, example, if we have a Buy One, Get One 50% OFF promotion running, these are instances where attempting to apply another discount code (what we call "stacking"), is not permitted.
We do allow loyalty rewards and discounts to be applied generally unless specified otherwise. These come in the form of Smile points through our system or inserts that come with purchases online.
If you're checking out on mobile, during checkout, you'll arrive at a screen where you can enter your billing and payment information. At the top of the screen, you will see "Show Order Summary" with a down arrow next to it. Shopify hasn't quite nailed this just yet, but you'll have to manually hit the down button to reveal the discount code field. Enter the code and you're good to go!