A failing discount code during a promotion is a small outage with outsized damage: the customers hitting it are exactly the ones ready to buy. The cause is almost always one of six things. Check them in this order.
Is the code expired, exhausted, or not started?
The three most common causes are the three most boring: the code's end date passed, its total usage limit was reached mid-promo, or its start date is set in a different timezone than you assumed. Check the discount's status and usage counter first; a code that "worked this morning" and stopped often just hit its cap.
Do the conditions actually match the cart?
Minimum purchase amounts, specific collections, customer-eligibility rules, and one-use-per-customer limits all fail silently from the customer's perspective: they see "code is not valid" with no explanation. Test with a cart that exactly matches your intended conditions before assuming a bug.
Is a combinability rule blocking it?
Shopify's discount combination settings decide whether codes stack with automatic discounts and each other. If an automatic discount is already applied, a non-combinable code will refuse, and customers read that as "broken." During big promos, audit which discounts are active simultaneously.
Is a discount or checkout app interfering?
Third-party discount, upsell, and bundle apps can override or conflict with native discounts, and when one of those apps has an outage or a bad deploy, discount behavior breaks with it. If the code fails only when a particular app's feature is in the cart, or the failure started without you changing anything, suspect the app layer and check the app vendor's status.
Is checkout itself degraded?
Rarely, discount validation fails because checkout infrastructure is having a moment: platform degradations like Shopify's brief June 2026 events can surface as checkout errors that look feature-specific. The full diagnostic for that path is in why is my Shopify checkout not working, and Shopify's live status settles it quickly.
Can customers report it faster than you can find it?
During promos, add a plain-text fallback in your promo emails ("if the code fails, reply to this email") so failures surface in minutes. And if your promotion depends on apps or platform features, monitoring those dependencies during the promo window is cheap insurance: a discount that fails for two hours on launch night is revenue you do not get back.
Start monitoring your stack free with StatusBird: 3 services, no credit card required. Pro is $29/month for all 84 services with SMS, email, Slack, Teams, and Discord alerts within 2 minutes of a status change.