下拉菜单不起作用。它只是向我展示了 <li> 里面的内容

否则我尝试制作一个下拉菜单，但我不知道为什么，它不起作用。它只是向我展示了里面的东西<li>

这是下拉菜单的 CSS

a, a:visited {
    text-decoration:none;
    outline:none;
    color:#54a6de;
}

a:hover{
    text-decoration:underline;
}

section, footer, nav{
    display: block;
}


/*----------------------------
    The Navigation Menu
-----------------------------*/


#colorNav > ul{
    width: 450px; /* Increase when adding more menu items */
    margin:0 auto;
}

#colorNav > ul > li{ /* will style only the top level li */
    list-style: none;
    box-shadow: 0 0 10px rgba(100, 100, 100, 0.2) inset,1px 1px 1px #CCC;
    display: inline-block;
    line-height: 1;
    margin: 1px;
    border-radius: 3px;
    position:relative;
}

#colorNav > ul > li > a{
    color:inherit;
    text-decoration:none !important;
    font-size:24px;
    padding: 25px;
}

#colorNav li ul{
    position:absolute;
    list-style:none;
    text-align:center;
    width:180px;
    left:50%;
    margin-left:-90px;
    top:70px;
    font:bold 12px 'Open Sans Condensed', sans-serif;
    
    /* This is important for the show/hide CSS animation */
    max-height:0px;
    overflow:hidden;
    
    -webkit-transition:max-height 0.4s linear;
    -moz-transition:max-height 0.4s linear;
    transition:max-height 0.4s linear;
}

#colorNav li ul li{
    background-color:#313131;
}

#colorNav li ul li a{
    padding:12px;
    color:#fff !important;
    text-decoration:none !important;
    display:block;
}

#colorNav li ul li:nth-child(odd){ /* zebra stripes */
    background-color:#363636;
}

#colorNav li ul li:hover{
    background-color:#444;
}

#colorNav li ul li:first-child{
    border-radius:3px 3px 0 0;
    margin-top:25px;
    position:relative;
}

#colorNav li ul li:first-child:before{ /* the pointer tip */
    content:'';
    position:absolute;
    width:1px;
    height:1px;
    border:5px solid transparent;
    border-bottom-color:#313131;
    left:50%;
    top:-10px;
    margin-left:-5px;
}

#colorNav li ul li:last-child{
    border-bottom-left-radius:3px;
    border-bottom-right-radius:3px;
}

/* This will trigger the CSS */
/* transition animation on hover */

#colorNav li:hover ul{
    max-height:200px; /* Increase when adding more dropdown items */
}


/*----------------------------
    Color Themes
-----------------------------*/


#colorNav li.green{
    /* This is the color of the menu item */
    background-color:#00c08b;
    
    /* This is the color of the icon */
    color:#127a5d;
}
#colorNav li.blue{      background-color:#53bfe2;color:#2884a2;}

这是 HTML 代码：

<nav id="colorNav">
    <ul>
 <li class="blue">
            <a href="#" class="fa fa-cogs"></a>
            <ul>
                <li><a href="#">Dropdown item 1</a></li>
                <li><a href="#">Dropdown item 2</a></li>
                <li><a href="#">Dropdown item 2</a></li>
                <li><a href="#">Dropdown item 2</a></li>
            </ul>
        </li>
</ul>
</nav>

这是它的外观：https : //imgur.com/n2AzTvJ