I have a problem with the add to cart button when I am not logged.
It does not appear in the categorie grid search but in the list view it does.
That happend with the single product view too.
I modify the file list.phtml from /app/design/frontend/royal/hijo/template/catalog/product
I remove the code but the problem still existing
@marina_córdoba May be there is a rule that only a product with price greater than a particular price would be purchase.
So try to change the price of that product and check if it is the problem.