高手帮忙，实在找不出原因

Extjs的combox组件没有办法加载数据
代码如下：
var deptselect = new Ext.data.Store({
reader:new Ext.data.JsonReader({
root:'root',
fields:['id','name']
}),
proxy:new Ext.data.HttpProxy({
url:'fbinfo.do?method=initDeptCombo'
})
});

var formpanel = Ext.create('InfoFB_FormPanel',{
layout:'form',
labelWidth:80,
labelAlign :'right',
items:[{
layout:'column',
frame:true,
items:[{
xtype:'combo',
id:'fbdept_id',
name:'fbdept',
hiddenName:'sub_fbdept',
columnWidth:.6,
allowBlank:false,
fieldLabel:'反馈科室',
emptyText:'--请选择--',
store:deptselect,
mode:'remote',
loadingText:'loading',
displayField:'name',
valueField:'id',
forceSelection:true,
typeAhead:true
}]
});

var infofbForm = function(){
var infofbWin = Ext.create('hosp.view.InfoFB',{
modal:true,
resizable :false,
constrain:true,
closeAction:'hide',
items:formpanel
});
infofbWin.show();
};
action部分：
public ActionForward initDeptCombo(ActionMapping mapping, ActionForm form,
HttpServletRequest request, HttpServletResponse response)
throws Exception {
response.setContentType("text/html;charset=utf-8");
PrintWriter out = null;
BrowseSvr bssvr = null;
String json = "";
try{
bssvr = new BrowseSvrImp();
json = bssvr.initDeptCombo();
out = response.getWriter();
out.write(json); //json值：{"root":[{"id":"500","name":"网络管理科"}]}
System.out.println(json);
}catch(MyException e){
throw new MyException("初始化加载部门信息失败！");
}
out.flush();
out.close();

return null;
}
json值：{"root":[{"id":"500","name":"网络管理科"}]}