There are three common reasons for this issue, covering both automatic and manual discount codes:
Shopify's automatic discount limit
Shopify allows a maximum of 25 automatic discounts. If that limit is reached, Discount Mixer can’t create or sync the necessary automatic discounts. In this case, you’ll likely see a banner in the app alerting you.
To resolve this, go to your Shopify Discounts section and delete unused automatic discounts. Then return to Discount Mixer and click Create automatic discounts. This will re-sync the necessary discounts and allow them to apply properly at checkout.
Missing or deactivated discounts
If you’ve accidentally deleted or deactivated any of the automatic discounts we created, the discounts may show on your storefront but fail to apply at checkout.
Make sure you always have at least the two Discount Mixer-generated automatic discounts visible and active in your Shopify admin.
If you've checked both of these and still experience issues, please contact our support team so we can assist you directly.