Only one time sorting happen

[sosick]

There 4 column in my gridview. When I click any column, sorting happen only once for that column. Where is am I wrong?Here is my code:\[code\]protected void Page_Load(object sender, EventArgs e){if(!IsPostBack){Bind();} }public void bind(){String connectionString = WebConfigurationManager.ConnectionStrings["ABConnectionString"].ConnectionString;SqlConnection con = new SqlConnection(connectionString); con.Open();DataSet ds = new DataSet();SqlDataAdapter da = new SqlDataAdapter("select * from Table1",con);try{da.Fill(ds, "Table1");GridView1.DataSource = ds.Tables["Table1"];Session["dttable"] = ds.Tables["Table1"];GridView1.DataBind();}catch (Exception ex){ex.ToString();}protected void GridView1_Sorting(object sender, GridViewSortEventArgs e){string sortExpression = e.SortExpression;string direction = string.Empty;SortDirection GridViewSortDirection = e.SortDirection;if (GridViewSortDirection == SortDirection.Ascending){GridViewSortDirection = SortDirection.Descending;direction = " DESC";}else{GridViewSortDirection = SortDirection.Ascending;direction = " ASC";}DataTable dt = (DataTable)Session["dttable"];dt.DefaultView.Sort = sortExpression + direction;GridView1.DataSource = dt;GridView1.DataBind();}public SortDirection SortDirection{get{if (ViewState["SortDirection"] == null){ViewState["SortDirection"] = SortDirection.Ascending;}return (SortDirection)ViewState["SortDirection"];}set{ViewState["SortDirection"] = value;}}}\[/code\]