jQuery UI不会显示自动完成菜单

我遇到了一个小问题。这里有这样的代码:

var suggestCallBack; // global var for autocomplete jsonp
$(document).ready(function () {
    $("#youtube-search").autocomplete({
        messages: {
            noResults: '',
            results: function() {}
        },
        source: function(request, response) {
            $.getJSON("https://suggestqueries.google.com/complete/search?callback=?",
                {
                  "hl":"en", // Language
                  "ds":"yt", // Restrict lookup to youtube
                  "jsonp":"suggestCallBack", // jsonp callback function name
                  "q":request.term, // query term
                  "client":"youtube" // force youtube style response, i.e. jsonp
                }
            );
            suggestCallBack = function (data) {
                var suggestions = [];
                $.each(data[1], function(key, val) {
                    suggestions.push({"value":val[0]});
                });
                suggestions.length = 5; // prune suggestions list to only 5 items
                response(suggestions);
            };
        },
    });
});

如果你按向上和向下箭头键，它将自动填充自动完成建议，但是它不会显示输入字段#youtube-search下面的菜单。知道哪里出错了吗?

提前感谢。:)