If you are sure products are in the product feed, the next thing to do is to check whether the products have image URLs. A product without any images associated with it won't appear in the app.
If the image URLs are in the feed, you can double check that those URLs are working properly.
Alternatively, the product could be out of stock and the app will hide the product automatically so that it does not appear in the appear.
If none of the options apply, please email firstname.lastname@example.org for help