Can't show Combobox in table

Today, I create Combobox-like have any time. And I don't know I can't show all item to choose.

You can look like:

Seem all items in ComboBox hidden behind the table.

HTML code:

<div class="btn-group open">
   <button type="button" class="multiselect dropdown-toggle btn btn-white btn-primary" 
          data-toggle="dropdown" title="None selected" aria-expanded="true">None selected 
          <b class="fa fa-caret-down"></b>
   </button>
   <ul class="multiselect-container dropdown-menu">
      <li><label class="checkbox">Checkbox</label></li>
      <li><label class="checkbox">
          <input type="checkbox" class="ace" value="0"><span class="lbl"></span> STT</label></a> 
      </li>
   </ul>
</div>

CSS code:

.btn-group {
    position: relative;
    display: inline-block;
    vertical-align: middle;
}

.multiselect-container {
    position: absolute;
    list-style-type: none;
    margin: 0;
    padding: 0;
}

.multiselect-container>li {
    padding: 0;
}

li {
    display: list-item;
}

.dropdown-menu {
    position: absolute;
    top: 100%;
    left: 0;
    z-index: 1000;
    display: none;
    float: left;
    padding: 5px 0;
    margin: 2px 0 0;
}

How to show all item of ComboBox in my case? Thanks.