如何调整菜单栏元素的响应行为

我正在尝试制作菜单。问题是，当我尝试减少600width下方的屏幕时，菜单由于响应性而消失。我希望菜单项像文本一样转到下一行。

如何实现此行为？

<section class="panel">
        <div class="panel-body" style="margin-top: -20px; min-width: 550px;">
            <div style="margin:0 auto;width: 550px;">
              <ul id="main-menu">
                    <li <?php echo $menuItem=='dashboard' ? 'class="current-menu-item"' : 'parent'?>><a href="index.php">Home</a></li>
                    <li <?php echo $menuItem=='submitForm' ? 'class="current-menu-item"' : 'parent'?>><a href="submissionForm.php">Submit Sale</a></li>
                    <li <?php echo $menuItem=='sales' ? 'class="current-menu-item"' : 'parent'?>><a href="saleslist.php">My Sales</a></li>
                    <li <?php echo $menuItem=='help' ? 'class="current-menu-item"' : 'parent'?>><a href="help.php">Help</a></li>
                    <li <?php echo $menuItem=='intro' ? 'class="current-menu-item"' : 'parent'?>><a href="intro.php?mode=view">Incentive Program Code</a></li>
                </ul>
             </div>
          </div>
</section>

我的 CSS 文件

@import url(http://fonts.googleapis.com/css?family=Open+Sans);
body {
    background-color: #eee;
    background-image: url(../images/patterns/light_toast.png);
    color: #666;
    font-family: 'Open Sans', sans-serif;
    font-size: 12px;
    margin:0px;
    padding:0px;
}
a {
    color: #23dbdb;
    text-decoration: none;
}
a:hover {
    color: #000;
}
ol, ul {
    list-style: none;
    padding:0px;
    margin:0px;
}
#wrap {
    margin: 0 auto;
}
.inner {
    margin: 0 auto;
    max-width: 940px;
    padding: 0 40px;
}
.relative {
    position: relative;
}
.right {
    float: right;
}
.left {
    float: left;
}
/* HEADER */
#wrap > header {
    background-color: #333;
    padding-bottom: 20px;
}
.logo {
    display: inline-block;
    font-size: 0;
    padding-top:15px;
}
#navigation {
    position: absolute;
    right: 40px;
    bottom: 0px;
}
#menu-toggle {
    display: none;
    float: right;
}
/* HEADER > MENU */
#main-menu {
    float: left;
    font-size: 0;
    margin: 10px 0;
}
#main-menu > li {
    display: inline-block;
    margin-left: 30px;
    padding: 2px 0;
}
#main-menu > li.parent {
    background-image: url(../images/plus-gray.png);
    background-size: 7px 7px;
    background-repeat: no-repeat;
    background-position: left center;
}
#main-menu > li.parent > a {
    padding-left: 14px;
}
#main-menu > li > a {
    color: #666;
    font-size: 13px;
    line-height: 14px;
    padding: 30px 0;
    text-decoration:none;
}
#main-menu > li:hover > a,
#main-menu > li.current-menu-item > a {
    color: #23dbdb;
}
/* HEADER > MENU > DROPDOWN */
#main-menu li {
    position: relative;
}
ul.sub-menu { /* level 2 */
    display: none;
    left: 0px;
    top: 38px;
    padding-top: 10px;
    position: absolute;
    width: 150px;
    z-index: 9999;
}
ul.sub-menu ul.sub-menu { /* level 3+ */
    margin-top: -1px;
    padding-top: 0;
    left: 149px;
    top: 0px;
}
ul.sub-menu > li > a {
    background-color: #333;
    border: 1px solid #444;
    border-top: none;
    color: #bbb;
    display: block;
    font-size: 12px;
    line-height: 15px;
    padding: 10px 12px;
}
ul.sub-menu > li > a:hover {
    background-color: #2a2a2a; 
    color: #fff;
}
ul.sub-menu > li:first-child {
    border-top: 3px solid #23dbdb;
}
ul.sub-menu ul.sub-menu > li:first-child {
    border-top: 1px solid #444;
}
ul.sub-menu > li:last-child > a {
    border-radius: 0 0 2px 2px;
}
ul.sub-menu > li > a.parent {
    background-image: url(../images/arrow.png);
    background-size: 5px 9px;
    background-repeat: no-repeat;
    background-position: 95% center;
}
#main-menu li:hover > ul.sub-menu {
    display: block; /* show the submenu */
}
@media all and (max-width: 700px) {
    #navigation {
        position: static;
        margin-top: 20px;
    }
    #menu-toggle {
        display: block;
    }
    #main-menu {
        display: none;
        float: none;
    }
    #main-menu li {
        display: block;
        margin: 0;
        padding: 0;
    }
    #main-menu > li {
        margin-top: -1px;
    }
    #main-menu > li:first-child {
        margin-top: 0;
    }
    #main-menu > li > a {
        background-color: #333;
        border: 1px solid #444;
        color: #bbb;
        display: block;
        font-size: 14px;
        padding: 12px !important;
        padding: 0;
    }
    #main-menu li > a:hover {
        background-color: #444; 
    }
    #main-menu > li.parent {
        background: none !important;
        padding: 0;
    }
    #main-menu > li:hover > a,
    #main-menu > li.current-menu-item > a {
        border: 1px solid #444 !important;
        color: #fff !important;
    }
    ul.sub-menu {
        display: block;
        margin-top: -1px;
        margin-left: 20px;
        position: static;
        padding: 0;
        width: inherit;
    }
    ul.sub-menu > li:first-child {
        border-top: 1px solid #444 !important;
    }
    ul.sub-menu > li > a.parent {
        background: #333 !important;
    }
}