DropDownList AutoPoskback does not work in UpdatePanel

There is a user control which implements a user control, this internal user control has an UpdatePanel and a DropDownList with the AutoPoskback property equal to true . The main user control has a button that performs the corresponding validations; but after the button event is executed it correctly shows the "*" character to indicate that it is required, but the DropDownList AutoPostback stops working; it works the second time to select an item from the drop-down list.

Internal user control

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="DdlVariable" EventName="SelectedIndexChanged" />
    </Triggers>
    <ContentTemplate>
        <div class="row">
            <asp:DropDownList runat="server" ID="DropDownList1" SkinID="skinDropDownList" Width="90%" AutoPostBack="true">
            </asp:DropDownList>

            <asp:RequiredFieldValidator ForeColor="Red" ID="RequiredFieldValidator1"
                runat="server" ControlToValidate="DropDownList1" Display="Dynamic"
                ErrorMessage="*"
                Font-Names="Verdana" Font-Size="7.5pt"></asp:RequiredFieldValidator>
        </div>
        <div class="row">
            <asp:TextBox ID="TextBox1" runat="server" MaxLength="3" SkinID="skinTextBoxContenido" />
        </div>
    </ContentTemplate>
</asp:UpdatePanel>

You have the main user control that implements the previous user control, and the button that releases the validations.

<div>
    <wuc:WucVariable runat="server" ID="WucVariable1" TituloLabelVariable="Variable 2" ValidationGroup="ModCombinada" UpdateMode="Conditional" />
</div>
<div>
    <asp:ImageButton ID="ImageButton1" runat="server" SkinID="skinImgAceptar" ValidationGroup="ModCombinada" />
</div>

The DropDownList has its event which all it does is a query to know a parameter, and finally register a script to launch the Javascript function. It is important to say that I am implementing the javascript ValidatorEnable function to enable or disable the validations of certain fields.

The main error is that if you launched the event click of the button; This executes the validations of both the controls that are the main user control and the controls that are in the internal user control. But after the validations are released, the autoposkback of the DropDownList stops working, of course when making the first attempt of the autoPostBack, after the second attempt the AutoPoskback works again. I do not know how to do so that I do not disable the autoPostBack at any time.