Problem from: https://github.com/rossp/django-menu#submenus

Sub menu not showing

<ul class="nav navbar-nav">{% load menubuilder %}
{% menu headernavigation %}
{% for item in menuitems %}
  <li>
    <a href="{{ item.url }}" title="{{ item.title|escape }}"{% if item.current %} class='current active'{% endif %}>{{ item.title }}</a>
  </li>
  {% if submenu %}<ul>
    {% for submenu in submenu_items %}
        <li>{{ submenu.title }}</li>
    {% endfor %}</ul>
  {% endif %}
{% endfor %}
</ul>

<!---- Or from: https://www.rossp.org/blog/menus/ ---->
{% load menubuilder %}
{% submenu %}
{% if submenu %}
  {{ submenu.name }}
  {% for item in submenu_items %}
    {{ item.title }}
  {% endfor %}
{% endif %}