Excel Application Events

In order to trap events of the worksheet that was dynamically generated, I define one class module and one code module like the following:

Class Module:
Public WithEvents xlapp as Application

Public Sub xlapp_SheetPivotTableUpdate(ByVal Wb As Workbook)
MsgBox " Testing Trap Events! "
End Sub


standard module:
Public xlApplication as new ClsAppEvents

Public Sub TrapApplicationEvents()
Set xlApplication.xlapp = Application
End Sub

I call TrapApplicationEvents from auto_open macro to trigger the worksheet's event automatically.

My problem is the pivottableupdate event can not be triggered automatically, I must run the TrapApplicationEvents macro manually every time to trigger the event.

Is there anybody know why?

Thanks