There’s multiple reason when offer will not show to your customer as below :

• If the app is disabled from Settings -> Checkout -> Post-purchase page.
• If none of the eligibility rule satisfied with ordered content.
• If all offers are expired or in disable mode.
• If selected offer product is out of stock.

Alternative payment methods:
The post-purchase page won’t be surfaced in the following scenarios,

• The customer chooses to check out with an installment service or a wallet service (such as Klarna, Affirm, AfterPay, Apple Pay, Amazon Pay, or Google Pay).
• The initial purchase was made with a gift card or any payment method other than a credit card.

Order creation delays:
In scenarios such as flash sales where the Shopify Platform is under extreme load, our system might optimize to capture orders but briefly delay the order creation step for a fast and seamless buyer experience. In these scenarios, post-purchase pages won’t be surfaced, even if the request for the post-purchase page was properly made.

Orders without a shipping address:
If the customer’s checkout results in the creation of an order without a shipping address, then you can’t add an offer to the order using post-purchase.