Created Issue: Ajax update panel not working [27375]

I have the three dropdownlist inside the update panel in my aspx page.But till it is giving the postbacks.Please help me in solving the issue.

.aspx page :
---------------
<tr>
<asp:ScriptManagerProxy ID="ScriptManager1" runat="server" />
<asp:UpdatePanel runat="server" ID="up1" UpdateMode="Conditional" >
<ContentTemplate>
<td class="xxx">
<asp:DropDownList ID="ddlFields" runat="server" EnableViewState="true"
AutoPostBack="true"> </asp:DropDownList>
</td>
<td class="xxx">
<asp:DropDownList ID="ddlOperator" runat="server" EnableViewState="true"
AutoPostBack="true"> </asp:DropDownList>
</td>
<td class="xxx" id="tdValue" runat="server">
<asp:DropDownList ID="ddlValue" runat="server" EnableViewState="true">
</asp:DropDownList>
</td>
</ContentTemplate>
</asp:UpdatePanel>
<td class="btnBarHeight">
<asp:Button ID="btnAdd" runat="server" Text="Add" ValidationGroup="ddd"/>
</td>
</tr>

And in my master page I have the

<act:ToolkitScriptManager ID="tsm" runat="server" ></act:ToolkitScriptManager>
after the <form> tag.

I trying to find why it is not working for a long time.Please help me.