AutoComplete Extender without webservice is not firing

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VB

ASP.NET

Ajax

Hi,
Iam using autocomplete extender in my page.But the function is not fired while execution.I am using master page in my application.
here is my Code

<asp:TextBox ID="txtDiagnosis"  runat="server"
                               Width="150px"  Font-Bold="True"
                               Font-Size="Medium" Font-Names="Courier New" style="margin-left: 15px"></asp:TextBox>
                           <asp:AutoCompleteExtender ID="txtDiagnosis_AutoCompleteExtender" runat="server"
                              MinimumPrefixLength="3" EnableCaching="false" CompletionSetCount="1" CompletionInterval="100" ServiceMethod="Casesheet.aspx/getDiagnosis"
                               TargetControlID="txtDiagnosis">
                           </asp:AutoCompleteExtender>

code behind

<System.Web.Script.Services.ScriptMethod(), _
 System.Web.Services.WebMethod()> _
    Public Function getDiagnosis(ByVal prefixtext As String) As List(Of String)
        Dim sqlstr As String
        Dim dtable As New DataTable

        Dim diaglist As New List(Of String)
        gappln_data.OpenConnection(gappln_data.appln_cn)

        sqlstr = "select DESCRIPTION from MAJORCODES where DESCRIPTION like '%" & prefixtext & "%"
        dtable = gappln_data.GetArray(sqlstr, gappln_data)
        gappln_data.appln_cn.Close()
        gappln_data.appln_cn.Dispose()
        If dtable.Rows.Count <> 0 Then
            Dim objDiagnosis As String
            For i = 0 To dtable.Rows.Count - 1
                objDiagnosis = dtable.Rows(i).Item(0).ToString
                diaglist.Add(objDiagnosis)
            Next
        End If
        Return diaglist
    End Function

Function getDiagnosis is not firing while execution