how to display selected value from dropdownlist with SqlDataSource

I have a DropDownList in my form in which I have configured a SqlDataSource. The value I select and save gets displayed perfectly in the GridView but when i click on edit button it goes back to the form page but doent display the selected item from the DropDownList.
This is my code:
 <td>                 Department Name: <span class="style2">*</span></td>             <td>                 <asp:DropDownList ID="DDL1" runat="server"                     DataSourceID="SqlDataSource1" DataTextField="departmentName"         & ...