Dropdown has lost css and refresh panel trigger cannot find item id inside gridview

The css dropdown style changes on page load. I tried several ways to solve the problem. also i cant find id dropdownlist in Update Trigger ... any suggestions to help me ... i am stuck on this .. thanks in advance ...

        <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
            <ContentTemplate>
                <asp:GridView runat="server" ID="gvDetails" AutoGenerateColumns="False" ShowHeaderWhenEmpty="True"
                    ForeColor="#333333" GridLines="None" ShowFooter="True" Width="855px">

                    <Columns>
                        <asp:BoundField DataField="SNo" HeaderText="S.N." ReadOnly="true" />
                        <asp:TemplateField Items Detail"
                            ControlStyle-Width="150px" ItemStyle-HorizontalAlign="center">
                            <ItemTemplate>
                                <asp:DropDownList ID="ddlItem" runat="server" ForeColor="Black" Height="35px" CssClass="chzn-select"
                                    Style="width: 210px;" Font-Bold="false" AutoPostBack="true" OnSelectedIndexChanged="ddlItem_SelectedIndexChanged">
                                </asp:DropDownList>
                            </ItemTemplate>
                            <FooterTemplate>
                                <asp:Label ID="lblTotalText" runat="server" Text="Total :"></asp:Label>
                            </FooterTemplate>
                            <ControlStyle Width="150px" />
                            <ItemStyle HorizontalAlign="Center" />
                        </asp:TemplateField>
                        <asp:TemplateField HeaderText="" ControlStyle-Width="0px" Visible="true">
                            <ItemTemplate>
                                <asp:TextBox ID="txtMRP" runat="server" Enabled="false" Visible="true" BorderStyle="None"
                                    BackColor="White" />
                            </ItemTemplate>
                            <ControlStyle Width="0px" />
                        </asp:TemplateField>
                        <asp:TemplateField HeaderText="" ControlStyle-Width="0px" Visible="true">
                            <ItemTemplate>
                                <asp:TextBox ID="txtAmount" runat="server" Enabled="false" Visible="true" BorderStyle="None"
                                    BackColor="White" />
                            </ItemTemplate>
                            <ControlStyle Width="0px" />
                        </asp:TemplateField>

                    </Columns>

                </asp:GridView>
            </ContentTemplate>
            <Triggers>
                <asp:AsyncPostBackTrigger ControlID="ddlItem" EventName="SelectedIndexChanged" />
            </Triggers>
        </asp:UpdatePanel>
        <br />

        <div style="margin-left: 50px; margin-top: 10px">
        </div>
        <br />
        <asp:Button ID="btnAddSalesOrder" runat="server" Text="Send Inquiry" OnClick="btnAddSalesOrder_Click"
            Style="margin-left: 30px; color: White; background-color: #088FCA;" OnClientClick="myClosure();"
            class="btn btn-info" />
        <asp:Button ID="btnPlaceOrder" runat="server" OnClick="btnPlaceOrder_Click" Text="Place Order"
            Style="margin-left: 30px; color: White; background-color: #088FCA;" OnClientClick="myClosure();"
            class="btn btn-info" />
        <asp:UpdateProgress ID="UpdtProgress" DisplayAfter="1" runat="server" EnableViewState="False">
            <ProgressTemplate>
                <div style="top: 0px; height: 120%; background-color: White; opacity: 0.75; filter: alpha(opacity=75);
                    vertical-align: middle; left: 0px; z-index: 999999; width: 120%; position: absolute;
                    text-align: center;">
                    <img src="Images/loader.gif">
                </div>
            </ProgressTemplate>
        </asp:UpdateProgress>
    </div>

<script type="text/javascript">
    $('.chzn-select').select2({
        no_results_text: 'Oops, nothing found!',
        allowClear: true,
        maximumSelectionSize: 1,
        width: '130px;'
    });