A discount isn't applying

Check these in order.

1. Is it enabled and active?

On Customizations, the function’s status should be Enabled.
In Shopify admin → Discounts, the discount should be Active, not Scheduled or Expired. If you ticked Set specific start date, check the date and time.

2. Do its display rules pass?

Switch the function to Apply to all checkouts to test. If it then applies, a rule is the problem.

Money rules use your store’s currency and are converted for the customer.
“is more than” is strict: use 49.99 for “$50 or more”.
Customer rules such as Customer tags only work for logged-in customers.
Only one cart attribute key per function. Two Cart attribute key, value rules with different keys don’t work together. The same goes for line attribute rules.
Use only rules offered for the function’s rule set. If a function stops working after you add a rule, remove that rule and save.
Order discounts: don’t use User is not authenticated as a customer. It currently stops the whole discount from applying.

3. Does it combine with other discounts?

When a customer has another discount, Shopify only applies both if each allows combining. Check Combines with order / product / shipping discounts on the function and the matching setting on the other discount.

Discounts in the same class, such as two product discounts on the same item, don’t stack unless you turn on stacking in the discount’s combinations settings.

4. Product discount set to “Checkout components” only

When Areas applied to has only Checkout components (upsells, cross-sells, etc.) ticked, the discount only applies to items added from a Checkout Buddy block. It won’t apply when:

The customer added the product from your store instead of from the block.
The app block has Disable custom attributes turned on.

Tick Cart as well to discount the product wherever it was added.

5. The block shows a discounted price, but the discount doesn’t apply

The price in a block is a preview based on the product discount’s settings. The discount’s display rules aren’t part of the preview. If the rules don’t pass, customers see the discounted price in the block, but pay full price at checkout. Most blocks also keep showing the discounted price after you disable the discount or it passes its end date.

Also check the two points in step 4.

6. The block shows the full price, but the discount applies

Blocks need a one-time metafield definition to show discounted prices. See show discounted prices in blocks.

Only automatic product discounts with Checkout components ticked in Areas applied to show a discounted price in blocks. Product discount codes, collection discounts, and order discounts apply at checkout without a preview.

7. Subscriptions aren’t discounted

For automatic product discounts, set Recurring cycle limit to 0 (all billing cycles) or the number of cycles you want. The default of 1 excludes subscription purchases.

8. The percentage is lower than expected

On a collection discount, Percentage is split across the matching items: 20% over three items gives about 7% each. Use Percentage (each product) to give the full percentage on every item.