子菜单未显示在Bootstrap中

.navbar-nav li:hover>ul.dropdown-menu {
  display: block;
}

.dropdown-submenu {
  position: relative;
}

.dropdown-submenu>.dropdown-menu {
  top: 0;
  left: 100%;
  margin-top: -6px;
}


/* rotate caret on hover */

.dropdown-menu>li>a:hover:after {
  text-decoration: underline;
  transform: rotate(-90deg);
}

.dropdown-menu>li:hover ul.dropdown-menu {
  display: inline-block;
  position: absolute;
  top: 2rem;
  left: 100%;
}

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css" integrity="sha384-/Y6pD6FV/Vv2HJnA6t+vslU6fwYXjCFtcEpHbNJ0lyAFsXTsjBbfaDjzALeQsN6M" crossorigin="anonymous">

<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.11.0/umd/popper.min.js" integrity="sha384-b/U6ypiBEHpOf/4+1nzFpr53nxSS+GLCkfwBdFNTxtclqqenISfwAzpKaMNFNmj4" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/js/bootstrap.min.js" integrity="sha384-h0AbiXch4ZDo7tp9hKZ4TsHbi047NrKGLO3SEJAg45jXxnGIfYzk4Si90RDIqNm1" crossorigin="anonymous"></script>

<!-- Menu Productos -->
<nav class="navbar navbar-expand-md navbar-light bg-light">
  <a class="navbar-brand pb-2" href="#">Navbar</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNavDropdown" aria-controls="navbarNavDropdown" aria-expanded="false" aria-label="Toggle navigation">
        <span class="navbar-toggler-icon"></span>
    </button>
  <div class="collapse navbar-collapse" id="navbarNavDropdown">
    <li class="dropdown">
      <a href="#" class="dropdown-toggle" data-toggle="dropdown">
        <i class="fas fa-project-diagram"></i> Productos <b class="caret"></b>
      </a>
      <ul class="dropdown-menu dropdown-menu--dark">

        <li>
          <a class="dropdown-item" [routerLink]='["/ConsoProductos"]'>
            <i class="fa fa-align-left" aria-hidden="true"></i> Consolidado
          </a>
        </li>
        <li class="dropdown-submenu--dark">
          <a class="dropdown-item dropdown-toggle" [routerLink]='["/RegistroProductos"]'>
         Registro de Productos
      </a>
          <ul class="dropdown-menu">
            <li>
              <a class="dropdown-item">
             Generación Nuevo Conocimiento
          </a>
            </li>

            <li>
              <a class="dropdown-item">
             Desarrollo Tecnológico e Innovación
          </a>
            </li>
            <li>
              <a class="dropdown-item">
                <i class="fa fa-align-left" aria-hidden="true"></i> Apropiación Social del Conocimiento
              </a>
            </li>
          </ul>
        </li>
      </ul>

    </li>

    <!-- Fin Menu productos -->
  </div>
</nav>