AJAX :: Update Pannel Is Not Updating The Controls On Dropdown Click In Aspx With Master Pages

the below code is in ASPX page and that page is using master pages and the update panel is not updating the controls ..

.it is working fine in normal ASPX page with out master pages.

Lables are populating from database

<asp:Content ID="CphPageContent" ContentPlaceHolderID="CphPageContent" Runat="Server">
<asp:ScriptManager ID="smSubmitReqOne" runat="server" />
<fieldset>
.
.
.
.
.
..
<tr>
<td>Application Requester:<span>*</span></td>
<td><asp:DropDownList ID="drpApplnRequester" runat="server" AutoPostBack="true" OnSelectedIndexChanged="drpdrpApplnRequester_SelectedIndexChange">
<asp:ListItem Value="0" Text="Select"></asp:ListItem>
</asp:DropDownList>
<asp:RequiredFieldValidator ID="rfvApplnRequester" runat="server" InitialValue="0" Display=None ControlToValidate="drpApplnRequester" ErrorMessage="Select the Application Requester" Visible=true ValidationGroup="vgSubmitRequestOne"></asp:RequiredFieldValidator>
</td>
<td>
<a href="AddApplnRequester.aspx" target="name"
onclick="modalWinRequester(); ">Add Application
Requester</a>
</td>
</tr>
<asp:UpdatePanel ID="upApplnReqOwner" runat="server" UpdateMode="Conditional" ChildrenAsTriggers="true">
<ContentTemplate>
<tr>
<td>
Application Requester E-mail:
</td>
<td>
<asp:Label ID="lblApplnReqEmail" runat="server" Text=""></asp:Label>
</td>
<td>
</td>
</tr>
<tr>
<td>
Application Requester Phone: </td>
<td>
</td>
<td>
</td>
</tr>
<asp:Label ID="lblApplnreqPhone" runat="server" Text=""></asp:Label>
</ContentTemplate>
<Triggers >
<asp:AsyncPostBackTrigger ControlID="drpApplnRequester" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>
protected void drpdrpApplnRequester_SelectedIndexChange(object sender, EventArgs e)
{
GPS.BusinessObjects.Users UserInfoRequester = new GPS.BusinessObjects.Users();
if (drpApplnRequester.SelectedValue.ToString() != "SELECT" && drpApplnRequester.SelectedValue.ToString() != "")
{
GPS.BusinessLogic.Users objUsers = new GPS.BusinessLogic.Users();
UserInfoRequester = objUsers.GetUserInfoByUserID(drpApplnRequester.SelectedValue.ToString());
lblApplnreqPhone.Text = UserInfoRequester.PhoneNumber;
lblApplnReqEmail.Text = UserInfoRequester.EmailId;
}
else
{ lblApplnreqPhone.Text = "";
lblApplnOwnerEmail.Text = "";
}
}