Archive by tag don't show custom post type

Hi, when I make an archive by tag (I have athaualpa theme but I make a tag.php page) the results shows only the “standard” post type, and not the custom post type (many and many posts) posts.
How can I display them?

the second (and less important) tihng is about the backend tag display.
when I enter into the tag management, the system display a large number of post, for a (i.e.) single tag, but clicking on that number, the list is empty (because of the system don’t consider the custom post type).
any suggestion for my two problems?
many many thanks
marco