DropdownList SelectedIndexChanged event not firing

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event

DropDownList

I m populating subcategory dropdown fom category dropdown.
but when i change category in category dropdown its "SelectedIndexChanged"event is not firing.here is my code:-

Markup:-

<td><asp:DropDownList ID="selectcategorydd" runat="server"
                class="styledselect_form_1" AutoPostBack="true"
                 AppendDataBoundItems="True"
                onselectedindexchanged="selectcategorydd_SelectedIndexChanged"></asp:DropDownList>
        </td>
        <td>
            <asp:RequiredFieldValidator ID="rfvc" runat="server"
                ControlToValidate="selectcategorydd"  ForeColor="Red" ErrorMessage="Select category">*</asp:RequiredFieldValidator>
            </td>
        </tr>
        <tr>
        <th valign="top" > SubCategory:</th>
        <td style="width:30%">
            <asp:DropDownList ID="selectscategorydd" runat="server" AutoPostBack="true" AppendDataBoundItems="True"
                class="styledselect_form_1"></asp:DropDownList>

CodeBehind:

protected void Fill_Category()
       {
           DataTable categorytbl = d.fill_category();
           selectcategorydd.DataSource = categorytbl;
           selectcategorydd.DataTextField = "CategoryName";
           selectcategorydd.DataValueField = "CategoryID";
           selectcategorydd.DataBind();

           selectcategorydd.Items.Insert(0, "--Select--");
       }



       protected void selectcategorydd_SelectedIndexChanged(object sender, EventArgs e)
       {
           int catid = int.Parse(selectcategorydd.SelectedValue);

           DataTable t1 = d.cascadesubcat(catid);
           selectscategorydd.DataSource = t1;
           selectscategorydd.DataTextField = "SubCategoryName";
           selectscategorydd.DataValueField = "SubCategoryID";
           selectscategorydd.DataBind();
           selectscategorydd.Items.Insert(0, "--Select--");
       }

Note:- viewstate is also true in page directive.