Here are some reasons that can potentially cause these issues:
There is a problem with the discount combination
If there is an issue with the discount combination, the easyPoints point discount might get rejected in the checkout page. Please check if any other discount is applied with the point redemption, and if so, that it allows combination with the Shopify's product discount coupon (This is what easyPoints uses).
The discount display is not accurate in the cart page
The discount display in the checkout page is always the source of truth, and is the most accurate information. However, the discount display in the cart might not be 100% accurate depending on your flow and the theme's set up. There is a high chance of it not being accurate when customers reload the page, or when they go back and forth between the checkout page and other pages. If something does not seem right, they can empty the cart, reload the page and start again.
Trying to use the express checkout link from the cart
The express checkout only works with point discounts when accessed from the checkout page. Please make sure customers do not access the link from the cart page if they wish to redeem points.
The store uses an old integration
easyPoints' custom theme integration feature might start experiencing some issues after a few years depending on the theme. If this is the case, we will guide you through a free of charge update.
Other reasons
If none of the reasons seem to be the case, please contact our support team (easypoints@lunaris.jp), and provide your store domain, details of the issue and your collaborator request code.