左侧的引导导航栏对象不会在移动设备上显示

由于某些原因，我无法弄清楚，当我在手机上时，放在左侧的引导导航按钮将不会显示（右侧的按钮分别是“ Home”和“ Link 2”精细）。具体来说，“注册”和“登录”按钮已损坏。

<nav class="navbar navbar-expand-lg navbar-light bg-light" style="margin-bottom:30px">
<a class="navbar-brand" href="#">Website name</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="#">Start <span class="sr-only">(current)</span></a>
        </li>
        <li class="nav-item">
            <a class="nav-link" href="#">Link 2 </a>
        </li>
    </ul>
</div>
<div class="navbar-collapse collapse w-100 order-3 dual-collapse2">
    <ul class="navbar-nav ml-auto">
        <li class="nav-item">
            <a class="nav-link" href="signup">Register</a>
        </li>
        <li class="nav-item">
            <a class="nav-link" href="login">Log in</a>
        </li>
    </ul>
</div>

谢谢！