Why isn't NHibernate deleting from the database?

UPDATE

public T SaveOrUpdate(T entity) 
{ 
    using (Session) 
    { 
        using (TransactionScope scope = new TransactionScope())
        { 
            Session.SaveOrUpdate(entity); scope.Complete(); 
        } 
        return entity; 
    } 
}

And this is absolutely correct... because your session FlushMode would most likely be:

session.FlushMode = FlushMode.Commit;

Pleae see more details here: Nhibernate Flush works commit doesn't

DELETE

But the poor sister Delete() is not fully supported as mighty Update()

public void Delete(T entity) 
{ 
    using (Session) 
    { 
        this.Session.Delete(entity); 
    } 
 } 

So, while even for Delete() the session FlushMode is still hooked on a transaction commit ... there is no transaction. And that's for sure (well most likely) the real reason (as suspected)

Summary, treat both, Update and Delete as the twins... and give them the same care - i.e. transaction