Why I can not bring to front, C# windows aplication

private void openFlToolStripMenuItem_Click(object sender, EventArgs e)
{
  Application.OpenForms["MainForm"].BringToFront();
}

private void positionsBalanceToolStripMenuItem_Click(object sender, EventArgs e)
{
  Positions_Balance p_b = new Positions_Balance();
  p_b.ShowDialog();
  p_b.BringToFront();
}

private void ordersToolStripMenuItem_Click(object sender, EventArgs e)
{
  Orders o = new Orders();
  o.ShowDialog();
  o.BringToFront();
}

private void symbolLookupToolStripMenuItem_Click(object sender, EventArgs e)
{
  Symbol_Lookup s_l = new Symbol_Lookup();
  s_l.ShowDialog();
  s_l.BringToFront();
}