EXTJS grid 排序 小弟我给显示的字段设置了 SORTABLE=TRUE 为什么有的字段可以排序有的字段不可以排序呢

EXTJS grid 排序 我给显示的字段设置了 SORTABLE=TRUE 为什么有的字段可以排序有的字段不可以排序呢
<script type="text/javascript">
  var IDS=new Array();
  Ext.onReady(function(){
  var roleCombox;
  var bbar;
  Ext.BLANK_IMAGE_URL = "<%=basePath%>js/extjs/resources/images/default/s.gif";
  Ext.QuickTips.init();
  Ext.lib.Ajax.defaultPostHeader+=";charset=utf-8";
  var cm=new Ext.grid.ColumnModel([
  {header:'角色名称',dateIndex:'ROLE_NAME',sortable:true},
{header:'创建人',dataIndex:'CREATE_USER_NAME',sortable:true},
{header:'创建时间',dataIndex:'CREATE_TIME',sortable:true}
  ]);
  //数据源的获取
  var store = new Ext.data.Store({
  reader: new Ext.data.JsonReader({
  totalProperty: "results",
  root: "items",
  id: "store"
  },
  Ext.data.Record.create(['ROLE_NAME','CREATE_USER_NAME','CREATE_TIME','ROLE_ID'])
  ),
  proxy : new Ext.data.HttpProxy({
  url : '<%=basePath%>system/role_findRole.action'
  });
  store.load({params:{start:0, limit:25}});
  var tbar=new Ext.Toolbar({
  name:'usertbar',
  id:'usertbar'
  });
  tbar.render('toolbar');
  var comboxRoleStore=new Ext.data.JsonStore({
  url:'<%=basePath%>system/role_findRoleByCom.action',
  root:'items',
  totalProperty: 'results',
  fields:[
  {name:'ROLE_ID', mapping:'ROLE_ID'},
  {name:'ROLE_NAME', mapping:'ROLE_NAME'}
  ],
  disableCaching:false,
  autoLoad:true
  });
  roleCombox=new Ext.form.ComboBox({
  xtype : 'combo',
  name:'ROLE_ID',
  id:'ROLE_ID',
  store: comboxRoleStore,
  displayField:'ROLE_NAME',
  valueField:'ROLE_NAME',
  emptyText:'请输入查询',
  width:160,
  listWidth:200,
  minHeight:100,
  editable:true,
  allowBlank:true,
  mode:'local', 
  typeAhead: false,
  loadingText:'loading...',
  hiddenName:'ROLE_NAME',
  triggerAction:'all',
   
  onKeyUp : function(e){
  var a=e.getKey();
  if(e.getKey()==e.SPACE||(parseInt(a)>48&&parseInt(a)<58)){
  this.store.baseParams = { query:this.getEl().dom.value };
  this.collapse();
  this.store.reload();
  }
  }
  });
  comboxRoleStore.load({params:{query:roleCombox.getRawValue(),start:0,limit:10}});
   
  var searchBus=new Ext.Toolbar.Button({text:'查询',handler:searchRole,cls:'x-btn-text-icon',icon: 'images/ico/sousuo.gif'});
  var roleCreate=new Ext.Toolbar.Button({text:'新增',handler:create,cls:'x-btn-text-icon',icon: 'images/ico/tianjia.gif'});
  var roleDelete=new Ext.Toolbar.Button({text:'删除',handler:deleteRole,cls:'x-btn-text-icon',icon: 'images/ico/shanchu.gif'});
  var roleModify=new Ext.Toolbar.Button({text:'修改',handler:modify,cls:'x-btn-text-icon',icon: 'images/ico/xiugai.gif'});
  var roleMenu=new Ext.Toolbar.Button({text:'角色配置菜单',handler:addRoleMenu,cls:'x-btn-text-icon',icon: 'images/ico/shezhi.gif'});
  var rolefun=new Ext.Toolbar.Button({text:'角色功能权限',handler:findmenurole,cls:'x-btn-text-icon',icon: 'images/ico/shenzhi.gif'});