Wicked PDF header not showing up

After already checking the Rails Gem repository for similar issues as well as Stack Overflow, I couldn't find an answer to my problem.

I'm trying to render a pdf using wicked_pdf within a Rails controller, but the header is not showing up, no matter what I do or which recommended solutions to similar issues I follow.

First and foremost, here is the development console output:

***************WICKED***************
  Rendering biddings/show.pdf.html.haml within layouts/pdf
  Rendered biddings/show.pdf.html.haml within layouts/pdf (0.7ms)
  Rendering biddings/header_pdf.html.haml within layouts/pdf_header
  Rendered biddings/header_pdf.html.haml within layouts/pdf_header (1.9ms)
"***************[\"/home/tommy/.rvm/gems/ruby-
 2.5.1@igalbids/bin/wkhtmltopdf\", \"-q\", \"--encoding\", \"UTF-8\",
  \"--javascript-delay\", \"500\", 
  \"--disable-internal-links\", \"--disable-external-links\", 
  \"--orientation\", \"Portrait\", \"--margin-top\", 
  \"50\", \"--margin-bottom\", \"25\", \"--header-html\", 
  \"file:////tmp/wicked_header_pdf20180801-27285-b8y5sg.html\", 
  \"--footer-right\", \"Página [page] de [topage]\",
  \"file:////tmp/wicked_pdf20180801-27285-1jfgdd7.html\", 
  \"/tmp/wicked_pdf_generated_file20180801-27285-1bkrvhx.pdf\"]***************"
  Rendering text template
  Rendered text template (0.1ms)
Sent data Licitación_2524.pdf (0.6ms)
Completed 200 OK in 2334ms (Views: 0.5ms | ActiveRecord: 64.4ms)

As you can see, both the header layout and its contents are being rendered and processed, however they don't make the final output PDF, and I don't know why! Look:

So, here's my controller code:

class Api::V1::Biddings::PdfBiddingsController < PdfController
  # JWT Authentication enforced
  before_action :authenticate_user!

  # GET /biddings/:id/pdf
  def show
    @bidding = scoped_collection.find(params[:id])

    authorize [:biddings, :pdf, @bidding]
    respond_to do |format|
      format.pdf do
        render(
          pdf: "#{Bidding.model_name.human}_#{@bidding.code}",
          disposition: "inline",
          orientation: "Portrait",
          template: 'biddings/show.pdf.html.haml',
          header: {
            html: {
              template: "biddings/header_pdf.html.haml",
              handlers: [:haml],
              layout: "pdf_header",
              formats: [:haml, :html]
            }
          },
          footer: {
            html: {
              handlers: [:haml],
              layout: "pdf",
              formats: [:haml, :html],
              encoding: 'UTF-8'
            },
            right: "#{I18n.t('pdf.page')} [page] #{I18n.t('pdf.of')} [topage]"
          },
          margin: { :top => 50, :bottom => 25},
          handlers: [:haml],
          layout: "pdf",
          javascript_delay: 500,
          encoding: 'UTF-8',
          show_as_html: false,
          disable_internal_links: true,
          disable_external_links: true) and return
      end
    end
  end

  protected
  def self.model
    Bidding
  end

  private
  def scoped_collection
    policy_scope([:biddings, :pdf, Bidding]).includes(:bidding_type, :client, :payment_condition, :price_list, :real_payment_condition, :sales_man, :user)
  end

  def records_per_page
    params[:per_page] || 10
  end
end

Nothing fancy, there you can see all the config options, pretty standard. Needless to say, the footer with the page numbering IS working fine (screenshot too long to show, but trust me). Can't say the same about the header.

Here's the PDF header layout file:

pdf_header.html.haml:

!!! 5
%html
  %head
    %meta{:content => "text/html; charset=utf-8", "http-equiv" => "content-type"}/
    = wicked_pdf_stylesheet_link_tag "bidding_pdf", media: :all
    = csrf_meta_tags
  %body.pdf
    = yield

and here the contents for the header "contents" per se:

header_pdf.html.haml:

Test text

Just plain text. I have a Linux 16.04 x64 OS, wicked_pdf (1.1.0), wkhtmltopdf-binary (0.12.4). How can I debug this?

For anyone else reaching here... it was a CSS issue. The header was there but "invisible" and no matter what margin I set on the render options it was a CSS issue. After starting the CSS from scratch, the header appeared! I could not debug it with the flag show_as_html: true because header and footer are not rendered in that mode, only the body.

If anyone reads this and happens to be in the same situation, use the search tool in the PDF document to find a word you know that's in the header. If it finds something but it's invisible, then you know you have a CSS problem. Also don't forget to check if you included in the html of the header the <!DOCTYPE html>. Thanks @joaolell for this.

Another thing to check, is that you have the version with patched qt of the wkhtmltopdf library (0.12.4 and above) that supports header and footer. Previous versions won't