ext动态生成checkbox不显示. 求解!

Question

'select':function(combo){
    var sub = Ext.getCmp("panel");
    sub.removeAll();
    Ext.Ajax.request({
        url:'xxx',
        method:'post',
        params:{"groupId" : combo.getValue()},
        success:function(res, o){
            var obj = Ext.util.JSON.decode(res.responseText);
            var items = new Array();
            for(var i = 0; i < obj.root.length; i ++){
                var sc = obj.root[i];
                items[i] = {
                    xtype:'checkbox',
                    checked: sc.ischecked == 0? false : true,
                    fieldLabel: sc.name,
                    value : sc.id,
                    inputValue: sc.id,
                    submitValue:true
                };
            }
            alert(items.length + " bcd");
            sub.add({
                xtype:'checkboxgroup',
                items:items,
            });
        }
    });
    sub.doLayout();
    sub.setHeight(200);
}
这是一个comboBox listener的回调 现在的效果就只改变了下高度. 第一次点combo的时候正常, 再点就显示不出来了...求解!ExtJSjavascriptCheckBox