Bootstrap multi选择在鼠标单击时不扩展

Question

检验代码：

<button type="button" class="btn dropdown-toggle bs-placeholder btn-default" data-toggle="dropdown" role="button" data-id="Relatedcategory_name" title="Nothing selected">
    <span class="filter-option pull-left">Nothing selected</span>
    &nbsp;
    <span class="bs-caret"><span class="caret"></span></span>
</button>

<div class="col-sm-5 col-md-5">
    <label>Related Category</label>
</div>
<div class="col-sm-7 col-md-7">
    <select multiple class="form-control selectpicker"  id="Relatedcategory_name" name="Relatedcategory_name[]">
        <option value="0">...Select Category...</option>
        <?php 
        if (isset($category_links)) {
            foreach ($category_links as $rs) {
                ?>
                <option value="<?php echo $rs->Category_Id; ?>"><?php echo $rs->Category_Name ?></option>
                <?php
            }
        }
        ?>
    </select>
</div>

检查中没有显示任何内容，当在标签或某些按键时到达时它会起作用。