Why are my Cart Upsell Suggestions not working?

This happens most likely because of how your theme works or the changes made to it, specifically on the Cart page. The requirement for the Cart Upsell Suggestions to show is for the products added to the cart (the cart line items) to have a URL in their title.
Our app can generate the Upsell Suggestion by using the URL as an identifier to know what is already added and missing. If the products in the cart don't have the URL, then the app does not have any way of knowing what products are there and what to recommend. Therefore the Upsell Suggestions will not show.
Unfortunately, this is an issue and a limitation coming from the theme that we cannot circumvent or fix, so this will require the help of the theme developer to add each product's URL correctly on the Cart page.
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