You may be running into an eccentricity in the BigCommerce platform.

BC does not allow cross-browser carts.

So, if a user created a cart in Chrome and was sent a recovery email but opened the link in Firefox then he will not see his cart populated. But if the same user opens the link in the same browser (Chrome in this case), it will display the cart properly.

If you run into a scenario that is different than this though, of course please reach out via the help chat in the bottom right corner!