Why is a product not appearing in the app but it is in the feed?
When adding new products to your feed, you first need to be aware of what time the feed is processed.
Once you know what time the feed is processed, you should add the products in before this time and allow for the feed to be generated so that it can be processed for the app.
Another thing to remember is that there may be caching which usually takes around 15-30 minutes to refresh before the products appear.
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.