Navbar Bootstrap and Dropdown

Question

I have a menu that contains submenus. When the resolution is mobile it only displays the menus and when I click on "products" it does not open the submenu it has on it, it is closed. What can be ?

<nav role="navigation" class="navbar">
                    <div class="navbar-header navbar-default">
                      <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#menu-collapse">
                        <span class="icon-bar"></span>
                        <span class="icon-bar"></span>
                        <span class="icon-bar"></span>
                      </button>
                    </div>
    
                    <div id="menu-collapse" class="collapse navbar-collapse menu">
                      <ul>
                        <li><a href="<?php echo base_url();?>">Home</a></li>
                        <li><a href="<?php echo base_url('sobre');?>">Sobre nós</a></li>
                        <li><a href="<?php echo base_url('produtos');?>">Produtos</a>
                        <ul>
                        <?php echo dropdown();?>
                        </ul>
                        </li>
                        <li><a href="<?php echo base_url('contato');?>">Localização & Contato</a></li>
                        <li><a href="<?php echo base_url('orcamento');?>"><i class="fa fa-pencil-square-o"></i> Orçamento</a></li>
                        <li class="hidden-md hidden-lg">
    
                          <br />
    
                          <form action="<?php echo base_url('buscar');?>" method="post">
                            <input type="text" class="form-control search"name="buscar" placeholder="Buscar...">
                          </form>
    
                        </li>
                      </ul>
                    </div>
    
                  </nav>