Alert is not showing after downloading ZIP file

Private Function DownloadZip(ByVal strFolderpath As String, ByVal strMissingSub As String) As Boolean
        Dim zip As New ZipFile()
        Dim DocumentsArray() As String
        Dim fInfo As FileInfo = Nothing
        Try
            If Not Directory.Exists(Server.MapPath(ConfigurationManager.AppSettings.Item("uploadfilepath").Trim()) 
           + "\" + Me.HProjectId.Value.ToString() + "\" + strFolderpath) Then
                Return True
            End If
            DocumentsArray = System.IO.Directory.GetFiles(Server.MapPath(ConfigurationManager.AppSettings.
            Item("uploadfilepath").Trim()) + "\" + Me.HProjectId.Value.ToString() + "\" + strFolderpath +    
          "\FinalReportLinear")
            If DocumentsArray.Length = 0 Then
                Return True
            End If
            Dim response As System.Web.HttpResponse = System.Web.HttpContext.Current.Response
            zip.AlternateEncodingUsage = ZipOption.AsNecessary
            zip.AddDirectoryByName("Files")
            For i = 0 To DocumentsArray.Length - 1
                fInfo = New FileInfo(DocumentsArray(i))
                If fInfo.Exists Then
                    Dim filePath As String = fInfo.FullName.ToString()
                    zip.AddFile(filePath, "Files")
                End If
            Next i
            response.Clear()
            Dim zipName As String = [String].Format("Zip_{0}.zip", DateTime.Now.ToString("yyyyMMMddHHmmss"))
            response.ContentType = "application/zip"
            response.AddHeader("content-disposition", "attachment; filename=" + zipName)
            zip.Save(response.OutputStream)
            response.Flush()
            response.End()
            Return True
        Catch ex As Exception
            Me.ShowErrorMessage("...........DownloadZip", ex.Message, ex)
            Return False
        Finally
            If strMissingSub <> "" Then
                ScriptManager.RegisterStartupScript(Me, typeof(Page), "UniqueID", "alert('Dossier File Not 
                Generated For Subject: " + strMissingSub + "')", True)
            End If
        End Try
    End Function