I am trying to use the Application events to check what sheets are being
activated by the user and then execute code accordingly. While this seems
to work fine on my PC, when I create an add-in and load it on another PC it
does not necessarily work (in some cases it does, in others it doesn't). In
those cases where it does not work the whole event is ignored. Anybody with
any experience on this?

The code I use is as follows:

'In the ThisWorkbook module
Dim WithEvents App As Application

Private Sub Workbook_Open()
Set App = Application
End Sub

Private Sub App_SheetActivate(ByVal Sh As Object)
Select Case Sh.Name
'code
End Select
End Sub