List items disappear after postback - asp.net

Question

I am having lots of problems with my dropdownlist. My dropdownlist controls disappear after postback.
The idea is that if in the drop down list I choose Spain, the ddlProvincia control shows the Spanish provinces. However, if I choose any country other than Spain, I should show the textbox "tbProvinciaNombre".
The problem is that when I select a country in the "ddlPais" control the list items of this control and "ddlProvincia" disappear completely.
Any help is welcome.
Thank you.
<asp:UpdatePanel ID="upProvinciaNombre" runat="server" UpdateMode="Conditional" class="col-md-6 form-group">
<ContentTemplate>
<!-- Provincia -->
<div id="panelDdlProvincia" runat="server" visible="true">
<asp:Label runat="server" AssociatedControlID="ddlProvincia">Provincia</asp:Label><span class="requerido">*</span>
<asp:DropDownList ID="ddlProvincia" runat="server" CssClass="form-control selectpicker" />
<asp:RequiredFieldValidator ID="rfvddlProvincia" runat="server" ErrorMessage="Seleccione una opción" CssClass="invalid-feedback" Text="*" InitialValue="none" ControlToValidate="ddlProvincia" ValidationGroup="ValidationButton"></asp:RequiredFieldValidator>
<ajax:ValidatorCalloutExtender ID="vceRfvddlProvincia" runat="server" TargetControlID="rfvddlProvincia" HighlightCssClass="error" CssClass="oculto" />
</div>
<!-- Provincia Nombre -->
<div id="panelTbProvinciaNombre" runat="server" visible="false">
<asp:Label runat="server" ID="lbProvinciaNombre" AssociatedControlID="tbProvinciaNombre">Provincia</asp:Label><span class="requerido">*</span>
<asp:TextBox ID="tbProvinciaNombre" runat="server" CssClass="form-control"></asp:TextBox>
<div>
<asp:RequiredFieldValidator ID="rfvTbProvinciaNombre" runat="server" CssClass="invalid-feedback" ControlToValidate="tbProvinciaNombre" Display="Dynamic" ValidationGroup="ValidationButton" ErrorMessage="Introduzca una provincia válida" />
<ajax:ValidatorCalloutExtender ID="vceRfvTbProvinciaNombre" runat="server" TargetControlID="rfvTbProvinciaNombre" HighlightCssClass="error" CssClass="oculto" />
</div>
</div>
</ContentTemplate>
</asp:UpdatePanel>
<!-- Pais -->
<asp:UpdatePanel ID="upPais" runat="server" UpdateMode="Conditional" class="col-md-6 form-group">
<ContentTemplate>
<asp:Label runat="server" AssociatedControlID="ddlPais">Pais</asp:Label><span class="requerido">*</span>
<asp:DropDownList ID="ddlPais" runat="server" CssClass="form-control selectpicker" AutoPostBack="true" />
<asp:RequiredFieldValidator ID="rfvddlPais" runat="server" ErrorMessage="Seleccione una opción" CssClass="invalid-feedback" Text="*" InitialValue="none" ControlToValidate="ddlPais" ValidationGroup="ValidationButton"></asp:RequiredFieldValidator>
<ajax:ValidatorCalloutExtender ID="vceRfvddlPais" runat="server" TargetControlID="rfvddlPais" HighlightCssClass="error" CssClass="oculto" />
</ContentTemplate>
</asp:UpdatePanel>
<!-- Políticas -->
<div class="col-md-12 form-group">
<div class="checkbox">
<asp:CheckBox ID="cbPoliticas" runat="server" />
<asp:Label runat="server" ID="politicas" AssociatedControlID="cbPoliticas">He leído y acepto la
<asp:HyperLink ID="hlkPoliticaPrivacidad" runat="server" Target="_blank" NavigateUrl="~/contenidos/privacidad.aspx" Text="política de privacidad" />
y el
<asp:HyperLink ID="hlkAvisoLegal" runat="server" Target="_blank" NavigateUrl="~/contenidos/avisoLegal.aspx" Text="aviso legal" />
del sitio web.
</asp:Label>
<div>
<asp:TextBox ID="tbFakeCv" runat="server" CssClass="oculto" />
<asp:CustomValidator runat="server" ID="cvCbPoliticas" ErrorMessage="Es obligatorio aceptar la política de privacidad" CssClass="error" Display="Dynamic" ControlToValidate="tbFakeCv" ValidateEmptyText="true" ValidationGroup="ValidationButton" />
<ajax:ValidatorCalloutExtender ID="vceCvCbPoliticas" runat="server" TargetControlID="cvCbPoliticas" HighlightCssClass="error" CssClass="oculto" />
</div>
</div>
</div>

Answer 1

The solution was really simple. In my case, I was using the class "selectpicker", but I assumed that this functionality was implemented in the .js file. But no, It was missing. Just add the following code in the .js file and everything is working like a charm.
Sorry. I just want to comment it because sometimes the frontend and backend are responsible of different people, and a error in one part could be a pain in the ass to the other member of the team.
// Selectpicker
if ($.fn.selectpicker) {
$('.selectpicker:not(.jQ_todos)').selectpicker();
$('.selectpicker.jQ_todos').selectpicker({ noneSelectedText: 'Todos' });
}