wp_insert_post – taxanomy does not get entered..

Creating a taxanomy:

function create_story_category(){
    register_taxonomy('story_type','story',array(
        'label' => __('Story Category') ,
        'assign_terms' => 'read',
        'edit_terms' => 'edit_posts'

    )) ;
}
add_action( 'init', 'ndjain_create_story_category' );

Adding a post via a form to a custom post_type.

$new_post = array(
            'post_title'    => $title,
            'post_content'  => $description,
            'tax_input' => array('story_type' => $story_cat ),
            'post_status'   => 'pending',
            'post_type' => 'story',
            'post_author' => $authorid
        );

        $pid = wp_insert_post($new_post);

It all works but the line 'tax_input' => array('story_type' => $story_cat ), seems to have no effect. The category is not added to the post

The var $story_cat is being correctly set btw.
manually adding the category from the dashboard works.

Not sure how to debug further

Help ?

Thanks