Custom Print Format is not displayed when click on Print button or PDF button

I have a custom Print Format made entirely with HTML. It is composed of a background image and a series of fields with their respective values.
At the time of printing, when you press any of the buttons to print or to convert to PDF, the print format is not displayed.
Exactly, when you click on the Print button, only the image I have in the background is displayed, and when I click on the PDF button nothing is shown, load a white page and nothing else.

By the way, I have updated wkhtmltopdf and setting host_name in site_config.json.

What I can be doing wrong?