wp_head() funktioniert nicht

// ### Code in der functions.php ###

<?php

function theme_slug_setup() {
   add_theme_support( 'title-tag' );
}
add_action( 'after_setup_theme', 'theme_slug_setup' );


function enqueue_my_custom_styles(){
  wp_enqueue_scripts( 'flexbox', get_template_directory_uri() . '/css/flexboxgrid.css', array(), 1.0.0, 'all');
}
add_action('wp_enqueue_scripts', 'enqueue_my_custom_styles');

function pix_debug_head() {
  echo '<!-- ####### Frisch eingefügt ###### -->';
}

add_action( 'wp_head', 'pix_debug_head' );

?>

//  ### Code aus der header.php ####

<!DOCTYPE html>
<html lang="de">
<head>
  <meta charset="utf-8">
  <?php wp_head(); ?>
</head>

<body>
  <header>
    <div class="container">
      <div class="row">
        <div class="col-lg-12">
          <a href="index.php">
            <?php  get_header_image(); ?>
          </a>
          <?php get_template_part('template_parts/navi'); ?>
        </div>
      </div>
    </div>

  </header>