Adding custom post types to archive

The problem could be related to how you register the custom post type.

Did you add the “taxonomies” argument in the register_post_type? If yes, is it a string? Try to convert it in an array of strings.

See the the example below…

    register_post_type(
        'custom_post_type_slug',
        array(
            'labels'               => array ( /* ... */ ),
            'public'               => true,
            /* ... */
            'taxonomies'           => array( 'custom_post_taxonomy_slug' ),
        )
    );