Jetta reverse boot

Greetings to everyone I just acquired an A4 VR6 jetta The only thing is the engine the box is equal to 4 cylinders and it has its details and the one I cannot give is that it bootes the reverse sometimes especially if it is forced the one that sold it to me (my carnal) says that I send to fix the box, that that is not, unless it has been bad, it is only in reverse the others they work well, it was well. If someone could guide me especially as a Czech that fails, I would really appreciate them