$wp_post_types doesn't contain custom post types

hi there, i’m adding in a drop down list of all post types to a meta box for a custom post type, my issue is that the $wp_post_types object does not contain any of my custom post types when in the admin section of my site, HOWEVER, when i print the same array on a page template all of my custom post types are there, my question is how to get the custom post types into the $wp_post_types object whilst in the admin section, so far there is no content on the subject, i have all of my post types working correctly, all are set to be public and everything is working with this object on the fornt end of the blog.