Custom styls of devise error messages display on sign-up page but don't appear on sign-in page

I made a helper_class named: devise_helper.rb:

module DeviseHelper
  def devise_error_messages!
    return '' if resource.errors.empty?

    messages = resource.errors.full_messages.map { |msg| content_tag(:li, msg) }.join
    html = <<-HTML
    <div class="alert alert-error alert-danger"> <button type="button"
    class="close" data-dismiss="alert">×</button>
      #{messages}
    </div>
    HTML

    html.html_safe
  end
end

for devise errors and made it custom style on application.css.scc and imported <%= devise_error_messages! %> inside both new.html.erb of session and new.html.erb of registration. But the problem that I have is when I test errors in sign-up page they are displayed beautifully above of my form but in sign-in page nothing happens. What is caused this problem? I also made a partial error messages:

 <% flash.each do |key, value| %>
  <div class="alert alert-<%= key %>">
    <a href="#" data-dismiss="alert" class="close">×</a>
    <ul>
      <li>
        <%= value %>
      </li>
    </ul>
  </div>
<% end %>

and rendered it in my application.html.erb