Seleted index is not changing in dropdownlist

See more:

C#

Hi, I have a problem that seems very common but I can't solve it. I am doing web forms project with C#, visual studio 2013.
I have used a dropownlist to select type of report. I have used 3 panels which are in updatepanel and the dropdownlist is outside of the Updatepanel. Initially I set visible property of each panel to false. I want to make one of the panels visible when user selects item from dropdownlist. The problem is when I change selection in dropdownlist, the SelectedIndexChanged event fires, but the selected index remains -1 every time. I have set autopostback to true. Following is my code.

<asp:DropDownList ID="ddlReportType" AutoPostBack="true" runat="server" OnSelectedIndexChanged="ddlReportType_SelectedIndexChanged" >
    <asp:ListItem Selected="True">-- Select which data you want to enter --</asp:ListItem>
    <asp:ListItem Value="Salse" Text="Salse">Salse Reports</asp:ListItem>
    <asp:ListItem Value="Inventory" Text="Inventory">Inventory Reports</asp:ListItem>
    <asp:ListItem Value="Finance" Text="Finance">Financial Reports</asp:ListItem>
</asp:DropDownList>
<br />
<asp:Label ID="Label1" runat="server" Text=""></asp:Label>
<br />
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>

         <asp:Panel ID="pnlProductSale"  runat="server">

             <asp:Label ID="lblCaptionSales" runat="server" Text="Total Product based sale"></asp:Label>
             <table>
                 <tr>
                     <td>
                         Select Product
                     </td>
                     <td>
                          <asp:DropDownList ID="ddlProduct" runat="server"></asp:DropDownList>
                     </td>
                 </tr>
                    </table>


         </asp:Panel>

           <asp:Panel ID="pnlInventory" runat="server">

             <asp:Label ID="lblInventory" runat="server" Text="Inventory"></asp:Label>
             <table>
                 <tr>
                     <td>
                         Select Product
                     </td>
                     <td>
                          <asp:DropDownList ID="ddlProductPnl2" runat="server"></asp:DropDownList>
                     </td>
                 </tr>

             </table>


         </asp:Panel>

        <asp:Panel ID="pnlCashFlow" runat="server">
        <asp:Label ID="lblCaption" runat="server" Text="Cashflow"></asp:Label>
<table>
    <tr>
        <td>
            Deposit at start up
        </td>
        <td>
            Cash on hand :   <asp:TextBox ID="txtStartAmt" Text="0" AutoPostBack="true" runat="server" ></asp:TextBox>
            Credit :  <asp:TextBox ID="txtStartCredit" Text="0" AutoPostBack="true" runat="server" ></asp:TextBox>
            Debit :  <asp:TextBox ID="txtstartDebit" Text="0" AutoPostBack="true" runat="server" ></asp:TextBox>
        </td>
    </tr>
    <tr>
        <td>
            <asp:ImageButton ID="imgSave" runat="server" />
        </td>
        <td>
            <asp:ImageButton ID="imgCancel" runat="server" />
        </td>
    </tr>
</table>

            </asp:Panel>


        </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="ddlReportType" />

    </Triggers>
    </asp:UpdatePanel>

following is the code in .cs file.

protected void Page_Load(object sender, EventArgs e)
    {
        if (Page.IsPostBack == false)
        {
            pnlCashFlow.Visible = false;
            pnlProductSale.Visible = false;
            pnlInventory.Visible = false;
        }
    }

    protected void ddlReportType_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (ddlProduct.SelectedIndex == 0)
        {
            Label1.Text = ddlProduct.SelectedValue;
            pnlCashFlow.Visible = false;
            pnlProductSale.Visible = false;
            pnlInventory.Visible = true;
        }
        else if (ddlProduct.SelectedIndex ==1)
        {
            Label1.Text = ddlProduct.SelectedValue;
            pnlCashFlow.Visible = false;
            pnlProductSale.Visible = true;
            pnlInventory.Visible = false;
        }
        else if (ddlProduct.SelectedIndex==2)
        {
            Label1.Text = ddlProduct.SelectedValue;
            pnlCashFlow.Visible = true;
            pnlProductSale.Visible = false;
            pnlInventory.Visible = false;
        }
        else
        {
            Label1.Text = ddlProduct.SelectedValue;
            pnlCashFlow.Visible = true;
            pnlProductSale.Visible = false;
            pnlInventory.Visible = false;
        }
    }

Please help me to solve this. Also tell me if I am using the wrong approach.