Jquery JWPlayer 6上一个和下一个按钮未显示

下面是我创建JW播放器的代码。理想情况下，如果我的播放列表包含多个元素，我应该能够看到“上一个”和“下一个”按钮。但由于某种原因，我看不到按钮。我错过什么了吗

      jwplayer(id_wrapper).setup({
            playlist: [{
            sources: [{
                file: 'http://content.bitsontherun.com/videos/bkaovAYt-52qL9xLP.mp4',
                height: 270,
            },{
                file: 'http://content.bitsontherun.com/videos/bkaovAYt-DZ7jSYgM.mp4',
                height: 720
            }],
            title: "Big Buck Bunny",
            description: "Big Buck Bunny is a short computer animated film by the Blender Institute.",
            image: 'http://d3el35u4qe4frz.cloudfront.net/bkaovAYt-480.jpg'
        },{
            file: 'http://content.bitsontherun.com/videos/3XnJSIm4-I3ZmuSFT.m4a',
            title: "Sintel (audio)",
            description: "Sintel is a short computer animated film by the Blender Institute."
        },{
            file: 'http://content.bitsontherun.com/videos/kaUXWqTZ-52qL9xLP.mp4',
            title: "Elephant's Dream",
            description: "Big Buck Bunny is a short computer animated film by the Blender Institute.",
            image: 'http://d3el35u4qe4frz.cloudfront.net/kaUXWqTZ-480.jpg'
        }],
        height : 360, //$(this).height() || '360',
        width : 640, //$(this).width() || '100%',
        listbar: {
                    position: "bottom",
                    size: 77
                }
      });

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只有播放列表（列表栏）不可见时，才能看到上一个/下一个按钮