我是 bootstrap 新手，遇到了两个大问题：

1. 我试图在菜单中包含一个下拉部分，但根本不起作用。

2. 当屏幕较小时，它会显示按钮菜单，但不起作用。

这是我从网站上救出的代码，应该可以完美运行，我不知道这是库错误还是我的行和列设置错误：

<div class="row menu" >
          <header class="barra">
          <nav class="navbar navbar-expand-lg navbar-dark bg-dark">
            <a class="navbar-brand" href="#">Navbar</a>
            <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
              <span class="navbar-toggler-icon"></span>
            </button>
          
            <div class="collapse navbar-collapse" id="navbarSupportedContent">
              <ul class="navbar-nav mr-auto">
                <li class="nav-item active">
                  <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
                </li>
                <li class="nav-item">
                  <a class="nav-link" href="#">Link</a>
                </li>
                <li class="nav-item dropdown">
                  <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
                    Dropdown
                  </a>
                  <div class="dropdown-menu" aria-labelledby="navbarDropdown">
                    <a class="dropdown-item" href="#">Action</a>
                    <a class="dropdown-item" href="#">Another action</a>
                    <div class="dropdown-divider"></div>
                    <a class="dropdown-item" href="#">Something else here</a>
                  </div>
                </li>
                <li class="nav-item">
                  <a class="nav-link disabled" href="#">Disabled</a>
                </li>
              </ul>
              <form class="form-inline my-2 my-lg-0">
                <input id="buscar" class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">
                <button class="btn btn-outline-warning my-2 my-sm-0" type="submit">Search</button>
              </form>
            </div>
          </nav>
        </header>
        </div>