为什么我的grid只显示地一条数据

Question

-----------------------------------------------------------js代码
  <script type="text/javascript">
     Ext.onReady(function(){   var pageSize = 16;
   
   var fields = ["UserId","UserName"];
   
   Uintstore = new Ext.data.Store({
       proxy:new Ext.data.HttpProxy({
           url: "DATA/Users.aspx",
           method: "POST"
       }),
       reader: new Ext.data.JsonReader({
           fields:fields,
           root:"data",
           id:"UserId",
           totalProperty:"totalCount"
       })
   });
   
   Uintstore.load({params:{start:0,limit:pageSize}});
   
   var sm = new Ext.grid.CheckboxSelectionModel({
dataIndex:"UserId"
   });
   
   var cm = new Ext.grid.ColumnModel([
        sm,{
        header:"编号",
dataIndex:"UserId",
//列不可操作
//menuDisabled:true,
//可以进行排序
        sortable:true
        },{
        header:"单位编号",
dataIndex:"UserName",
        sortable:true
        }]);
        
   var grid = new Ext.grid.GridPanel({
       id:"UnitGrid",
       store:Uintstore,
       cm:cm,
       sm:sm,
       viewConfig:{
columnsText:"显示/隐藏列",
sortAscText:"正序排列",
forceFit: true,
sortDescText:"倒序排列"

},
renderTo:document.body
   })
     });    </script>
--------------------------------------------------------------返回的json
{totalCount:5,success:true,error:"",singleInfo:"",data: [{"UserId":"1","UserName":"香味 "},{"UserId":"2","UserName":"响亮 "},{"UserId":"3","UserName":"大为 "},{"UserId":"4","UserName":"洋溢 "},{"UserId":"5","UserName":"无误 "}]}
------------------------------------------------------刚刚学习EXTJS