Jquery-ui in my plugin – jQuery("ul").sortable is not a function

I’m trying to use Jquery-UI in an admin page in my plugin, and I’m getting this error when I try to use a jquery-ui function:
jQuery(“ul”).sortable is not a function
Here is how I’m loading the scripts:

function list_ads_scripts() {
    wp_enqueue_script('jquery-ui-core');
    wp_enqueue_script('jquery-ui-dropable');
    wp_enqueue_script('jquery-ui-dragable');
    wp_enqueue_script('jquery-ui-selectable');
    wp_register_script('list_ads', WP_PLUGIN_URL.'/ad_manager/js/list_ads.js', array('jquery', 'jquery-ui-core'));
    wp_enqueue_script('list_ads');
    }

And here is my javascript file (list_ads.js):

jQuery.noConflict();
jQuery(document).ready(function(){
  alert('test');
  jQuery('ul').sortable();
});

The alert appears succesfuly, I just get the error with the sortable function.