Easy Upsell uses the Shopify Post-purchase extension. There are some known limits that you might want to review.
1. Upsells will not be available if the customer has made their order in a different currency to your shop's currency.
2. Upsells may not be displayed if the Shopify server is under significant load.
3. Payment methods other than credit cards are not supported, including Pay Pal and gift cards.
4. Products offered on subscription are not currently supported in an upsell.