Ajax ComboBox rendering ListItems correctly but not displaying

Here ddlSchool is the ajax Combo box

<code>
ddlSchool.DataSource = dt;
                             ddlSchool.DataTextField = "SchoolName";
                             ddlSchool.DataValueField = "Id";
                             ddlSchool.DataBind();
                             ddlSchool.Items.Insert(0, new System.Web.UI.WebControls.ListItem(" --Select-- ", "0"));
                             ddlSchool.SelectedValue = "0";</code>

when I inspect it renders the value but not displaying ,below code is only available during inspect

<code><ul id="ContentPlaceHolder1_ddlSchool_ddlSchool_OptionList"class="ajax__combobox_itemlist" style="visibility: hidden; z-index: 1000; overflow-x: hidden; overflow-y: auto; width: 416px; position: absolute; height: 425px; left: 293px; top: 1458px; display: none;">
<li>Maharishi Vidhyala</li>
<li>Little Kindom</li>
<li>Malaiar Manram</li>
<li>Sparkle dots</li>
<li>KTC nursery school</li>
</ul></code>

Please help me to get the ListItems