NHibernate collections not persisted

I have a rather strange error with NHibernate. I had a bug with ISession that was split between threads and solved by supplying my own ADO.NET connection, like this:

            IDbConnection connection = new SqlConnection(ApplicationConfiguration.ConnectionString);
            connection.Open();

            ISession session = _sessionFactory.OpenSession(connection);
            session.FlushMode = FlushMode.Commit;

            return session;

      

        
        
        
      

    

Now my application works, but all objects with collections are saved in the database without their collections. for example, a car is said to have a tire list. then I create a car and then I create a list of tires based on tires already in the database. saving the vehicle object will save the vehicle not in the list!

any help on what i am doing wrong? I am using NHibernate 2.0 and I am calling Session.Flush () and Transaction.Commit ().

greetings.