A client purchased a product, but the "My Products" menu does not appear on his client website and he cannot access the product. Why is that?

The problem was that initially you've made the access to the product content available from December, while we are currently in November, and then changed it to be from November. When the client purchased the product, he purchased it with the initial December access date and that is why he could not access the product. That's the way it stayed even after you modified the date to November, because what the client purchased and still has is a copy of the product with the initial settings (content availability starting from December). The action of changing the content availability in the "parent" product is not reflected in the purchased product copy. So, please keep that in mind when working with products.

In such cases we would need to modify the product access date from the database, so that the buyers can get access to the product content sooner (November in our example).