Why is my discount code not applying at checkout?

The usual culprits: a discount combinations rule blocking it from stacking, a Shopify Function that wasn’t deployed or has a logic bug, a discount that only targets the cart but not the checkout, or an old Script Editor script that stopped running after the Checkout Extensibility move. Errors like “This code isn’t valid” or a discount that shows in cart then vanishes at payment point to one of these. We trace which layer is dropping it and fix it.