In SqlDataSource, while filtering data in Dropdownlist, the gridview is not loaded with data?

Question

In Asp.net SQLdatasource, I applied filter expression and filter parameter. While filtering a data in the dropdownlist, the Gridview should be loaded based on the filteration in the dropdownlist. But Gridview is not loaded with data and gridview structure as follows:
     <tr>

                <td>

                    <asp:DropDownList ID="DropDownList1" runat="server" AppendDataBoundItems="True"

                        DataSourceID="sqlDataSourceLocation" DataTextField="FirstName" DataValueField="ID"

                        AutoPostBack="True">

                    </asp:DropDownList>

                </td>              

            </tr>

            <tr>

                <td>

                    <asp:GridView  ID="GridView1" ShowHeader="False" AllowPaging="True"  runat="server"

                         DataSourceID="SqlDataSource1" AutoGenerateColumns="False"

                        DataKeyNames="ID">

                        <Columns>

                            <asp:BoundField DataField="ID" HeaderText="ID" InsertVisible="False"

                                ReadOnly="True" SortExpression="ID" />

                            <asp:BoundField DataField="FirstName" HeaderText="FirstName"

                                SortExpression="FirstName" />

                            <asp:BoundField DataField="LastName" HeaderText="LastName"

                                SortExpression="LastName" />

                            <asp:BoundField DataField="Location" HeaderText="Location"

                                SortExpression="Location" />

                        </Columns>

                    </asp:GridView>

                </td>            

            </tr>

            <tr><td> <asp:SqlDataSource ID="SqlDataSource1" runat="server"

                        ConnectionString="<%$ ConnectionStrings:TestDBConnectionString3 %>"

                        SelectCommand="SELECT [ID],[FirstName],[LastName],[Location] FROM [Employees]"

                        FilterExpression="[FirstName] = '{0}'">

                        <FilterParameters>

                            <asp:ControlParameter ControlID="DropDownList1"

                                Name="FirstName" PropertyName="SelectedValue" />

                        </FilterParameters>                       

                    </asp:SqlDataSource>

                      <asp:SqlDataSource ID="sqlDataSourceLocation" runat="server"

            ConnectionString="<%$ ConnectionStrings:TestDBConnectionString3 %>"

            SelectCommand="SELECT Distinct [FirstName], [ID] FROM [Employees]"></asp:SqlDataSource></td></tr>

        </table>

    Please help me regarding this.

Thanks a lot