ASP.Net DropDownList using Modal Popup Extender - Selected value getting lost

Hey,

I am unable to get the selected value of the DropDownList.  When the user selects a value and clicks OK, the value that was loaded as the default selected value with an index of 0 is available in code behind, not the newly selected value.  See the attached code for the dropdownlist in the Modal Popup form.  Thanks 

<div class="well" style="margin: 5px">
            <asp:Label ID="lblPlanOptAdd" runat="server" AssociatedControlID="ddlPlanOptAdd">Plan Option:</asp:Label>
                                                
            <asp:DropDownList ID="ddlPlanOptAdd" runat="server" CssClass="form-control" OnSelectedIndexChanged="PlanOptAdd">
            </asp:DropDownList>            
                        <asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" ControlToValidate="ddlPlanOptAdd" 
                             CssClass="failureNotification" ErrorMessage="Plan Option is required." ToolTip="Plan Option is required." 
                             ValidationGroup="UserCodeValidationGroup">*</asp:RequiredFieldValidator>
                                         <br />
                     <asp:Label ID="lblBusType" runat="server" AssociatedControlID="ddlBusType">Business Type:</asp:Label>
                        
            <asp:DropDownList ID="ddlBusType" runat="server" CssClass="form-control">
            </asp:DropDownList>                        
                        <asp:RequiredFieldValidator ID="RequiredFieldValidator3" runat="server" ControlToValidate="ddlBusType" 
                             CssClass="failureNotification" ErrorMessage="Plan Option is required." ToolTip="Business Type is required." 
                             ValidationGroup="UserCodeValidationGroup">*</asp:RequiredFieldValidator>
                                         <br />
                     <asp:Label ID="lblBusTypeEffDt" runat="server" AssociatedControlID="txtBusTypeEffDt">Effective Date:</asp:Label>
                        <br />
                       <asp:TextBox ID="txtBusTypeEffDt" runat="server" CssClass="form-control"></asp:TextBox>
                       <asp:CalendarExtender ID="CalendarExtender3" 
                runat="server" Enabled="True" TargetControlID="txtBusTypeEffDt">
            </asp:CalendarExtender>                        <br />
                     
                        <asp:Label ID="lblBusTypeTermDt" runat="server" AssociatedControlID="txtBusTypeTermDt">Termination Date:</asp:Label>
                        
                        <asp:TextBox ID="txtBusTypeTermDt" runat="server" CssClass="form-control"></asp:TextBox>
                        <asp:CalendarExtender ID="CalendarExtender4" 
                runat="server" Enabled="True" TargetControlID="txtBusTypeTermDt">
            </asp:CalendarExtender>            <br />            
            <asp:Label ID="lblParStatus" runat="server" AssociatedControlID="cbxlParStatus">Participating Status Selection(s):</asp:Label>
                       
            <asp:CheckBox ID="cbxSelAll" runat="server" Text="Select All" 
                OnCheckedChanged="PS_SelAll" AutoPostBack="True" Visible="False" />
            <br />           
            <asp:CheckBoxList ID="cbxlParStatus" runat="server" RepeatColumns="2" 
                RepeatDirection="Horizontal" RepeatLayout="Table" Width="500px" 
                CssClass="well">            
             </asp:CheckBoxList>                       <br />
                             
            <asp:Label ID="Label6" runat="server" Text=""></asp:Label>
            <br />
                             <asp:Button ID="Button1" runat="server" 
                CssClass="btn btn-primary" Text="OK" 
                        ValidationGroup="UserCodeValidationGroup" OnClick="BenCodeMap_Click" />
                        
                        <asp:Button ID="Button2" runat="server" CssClass="btn btn-primary" Text="Cancel" 
                        ValidationGroup="UserCodeValidationGroup" />
                               </div>