Delete an recurrent appointment with a "deleted" exception

I'm creating an addin for Outlook using C#.NET in Visual Studio .NET
via the Extensibility.IDTExtensibility2 interface. The addin, among
other things, basically sinchronize appointment with a SQL Server
database via a Web Service.
My problem comes as follows:
1. Interactivelly create a recurrent appointment.
2. Interactivelly delete a repetition of the appointment. An exception
with property Deleted is created in the RecurrencePattern object.
3. Programatically delete the appointment. Here I got an exception:
"Outlook can't delete this element".
Note: If I restart Outlook and try step 3 then I don't have any problem
and the appointment is deleted correctly.