Coolite里的GridPanel手动动态的添加列的问题？

一个小例子 可是却总是不成功..请高手赐教
前台:
<ext:ScriptManager ID="ScriptManager1" runat="server" />
<ext:Store ID="GridPanel1Store" runat="server"></ext:Store>
<ext:GridPanel ID="GridPanel1" Height="300" runat="server" StoreID="GridPanel1Store">
</ext:GridPanel>
<ext:Button ID="button1" Text="Show" runat="server">
<AjaxEvents>
<Click OnEvent="Button_Click"></Click>
</AjaxEvents>
</ext:Button>
后台:
protected void Page_Load(object sender, EventArgs e)
{
     //if(!IsPostBack)
    //    DataInit();
}
protected void Button_Click(object sender,AjaxEventArgs e)
{
    DataInit();
}
protected void DataInit()
{
    RecordField field = new RecordField("Semester", RecordFieldType.String);
    GridPanel1Store.AddField(field, GridPanel1Store.Controls.Count);
    Coolite.Ext.Web.ColumnBase column1 = Coolite.Ext.Web.Column();
    column1.Header = "学期";
    column1.Sortable = true;
    column1.DataIndex = "Semester";
    //GridPanel1.AddColumn(column1);方法报错
    GridPanel1.ColumnModel.Columns.Add(column1);
    //生成数据
    DataTable datatable = new DataTable();
    datatable.Columns.Add("Semester");
    for (int i = 0; i < 5; i++)
    {
        DataRow row = datatable.NewRow();
        row["Semester"] = "abc" + i;
        datatable.Rows.Add(row);
     }
     //绑定数据
     GridPanel1Store.DataSource = datatable;
     GridPanel1Store.DataBind();
}问题:当方法在Page_Load里调用时会被加载并显示,但写在按钮的Click事件里却只是有加载没有显示...怎么回事?