There are a few restrictions for products to be indexed when the indexing type is set to "auto".
The first one is if you have the Yoast plugin installed and activated. If yes, you need to look at the visibility that the product has in the Yoast option, as Doofinder also rely on it to index a product that is set to auto. There are times when you can see that the product is on auto, but if you go to your yoast options, it is as not visible:
In case Yoast does not apply, you also have to check that the corresponding post or product is not password protected. If it is, it will not be public and therefore not accessible.
Finally, the product must have a public status.