There are multiple factors that might prevent the post-purchase one-click upsell from appearing for the customer, some of which are global limitations imposed by Shopify.
Post-purchase offers will not render if the order was placed with:
Your store has multi-currency enabled and the order was made in a currency other than your "store currency".
You don't have Kaching Upsell enabled as your post-purchase page app in your Checkout settings.
Total price less than 0.50$
The order was paid for using a Gift Card, COD or it was a $0 (free) order.
The original order contains a subscription product, and the upsold product is also a subscription product.
Your product has inventory tracked and the inventory amount is zero or less.
Your product is in an order for local delivery.
The Shopify Platform is under extreme load.
To learn more about global Shopify limitations for post-purchase apps, please refer to this official Shopify article