ScriptManager.RegisterClientScriptBlock not working

As you can see in my function below, I did use ClientScript.RegisterClientScriptBlock and it worked fine.

I have now added an update panel to one of my pages so need this javascript alret function to work in there so have created an ajax version.

What do I need to do to:
ScriptManager.RegisterClientScriptBlock(Me.Page, Me.GetType, "DisplayAlertaj", script, True)

As it isn't currently doing anything?

Thanks in advance

Public Sub JavascriptAlertAjax(ByVal message As String)
        Try
            Dim script = "<script language='JavaScript'> "
            script += "DisplayAlert('" & message & "')"
            script += "</script>"
            'ClientScript.RegisterClientScriptBlock(Me.GetType, "DisplayAlert", script)
            ScriptManager.RegisterClientScriptBlock(Me.Page, Me.GetType, "DisplayAlertaj", script, True)
        Catch ex As Exception
            Throw
        End Try
    End Sub