Initial text:
Hello, I guess you've solved this issue in the meantime, could you pretty please share how ?
I am facing the same problem, have a lot of new products, reseted the New Products module, cleared cache, disabled cache, all those.
Thank you.
Update, after finding my "problem":
Without going trough all my struggle, "my problem" was that I was using the Duplicate product button because the main difference between the products is the title and picture so I couldn't be bothered to write all and everything again and most of all, the Combinations (as there are a few, same for all).
So, in duplicating one product, Prestashop 8.1.2 also duplicates the product_add column (date) value in product_shop and product MySQL tables thus all duplicated products were the same "age" and this was the reason they would stop showing in New Products module.
I'm not sure about this implementation from Prestashop guys, should a duplicated product be considered same age as the original or a new one ? They decided: same age.
Is there a workaround for this instead of going into the database and updating product_add value ?
Maybe I am doing/thinking it all wrong but I really don't wanna manually enter all product details when, as mentioned, the only difference is picture and name. For me, for future operations, I'll just modify the source code to use current date. Peace.