asp.net - DropDownList in Bootstrap XS do not open -

i have asp application , problem dropdownlist. not open on xs mobile devices:

<div class="row">     <div class="col-xs-4 col-sm-2 col-md-1">         <!-- anzahl zeilen dropdownlist -->         <asp:dropdownlist id="ddlusergridviewrows" runat="server"             autopostback="true"             cssclass="form-control font-13"             data-style="btn-primary">             <asp:listitem value="5"> 5 </asp:listitem>             <asp:listitem value="10"> 10 </asp:listitem>             <asp:listitem value="15"> 15 </asp:listitem>             <asp:listitem value="20"> 20 </asp:listitem>             <asp:listitem value="30"> 30 </asp:listitem>             <asp:listitem value="50"> 50 </asp:listitem>         </asp:dropdownlist>     </div>     <div class="col-xs-6 col-sm-2 font-13 hidden-xs" style="padding-top: 6px;">         zeilen pro seite     </div>     <div class="col-sm-offset-4 col-sm-4 col-md-offset-5 col-lg-offset-6 col-lg-3">         <!-- unit dropdownlist -->         <asp:dropdownlist id="ddlunitlist" runat="server"             autopostback="true"             cssclass="form-control font-13"             data-style="btn-primary" />     </div> </div> 

this works on lg, md , sm screensize (bootstrap). resize browser xs screen size no longer open first dropdown list. idea?!?

use bootstrap dropdown think solve problem. use class="dropdown" main div , assign class="dropdown-menu" dropdown list item.

for more details study : http://getbootstrap.com/components/#dropdowns