[JerTM]

First, it was covered by the CPO warranty. Second I would find another dealer, especially after your inability to contact them (Do you really trust them to perform an effective and timely repair?) Third, I'd let GM know the issues this dealer is giving you regarding the repairs and inability (unwillingness) to communicate with you or schedule a reasonable time to repair the vehicle. My professional advice is not to contact that dealer any further after its fixed and find one that does want you as a customer.