Ztrader

Hi guys, So i bought an Audi A5 2011 TDI with automatic transmission for two weeks ago from a private seller. He had recently taken service on the transmission and had it diagnosed. No problems showed up. Today I went to the store and when I came back it wouldnt go to reverse. Only drive worked. Took a new diagnose on it and no faults showed up. What may the issue be? Kind a sucks that I bought the car two weeks ago and this happens.