Pageindexchanging not refreshing until other event...

protected void LastThirtyDaysReportGrid_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
    GetThirtyDay(null, null);
    LastThirtyDaysGrid.PageIndex = e.NewPageIndex;
    LastThirtyDaysGrid.DataBind();
}

  protected void GetThirtyDay(object sender, EventArgs e)
  {
      GenerateReport("LastThirtyDaysExpenses", LastThirtyDaysGrid, "Last Thirty Days' Expenses");
  }



private void GenerateReport(string storedProcName, GridView reportGridview, string reportTitle)
  {
      ClearGridViews();

      try
      {
          using (var conn = new SqlConnection(connectionString))
          {
              var command = new SqlCommand();
              command.CommandText = storedProcName;
              command.Connection = conn;
              command.CommandType = CommandType.StoredProcedure;

              var dataAdapter = new SqlDataAdapter(command);
              var dataTable = new DataTable();
              dataAdapter.Fill(dataTable);

              reportGridview.DataSource = dataTable;
              reportGridview.DataBind();
              ReportHeader.Text = reportTitle;
          }
      }
      catch (Exception exp)
      {
          string help = exp.ToString();
      }
  }