No Country For Old Men audio issues

[JCarr2]

When I select the 5.1 dolby digital audio option on this blu-ray disc, my receiver (older Kenwood model with dolby digital and pro logic capabilities) won't output 5.1 surround. It keeps combining the rear two speakers into one simulated speaker. Why is that? Whenever I choose 5.1 on my other blu-ray discs it works fine. Could there be a problem with the disc itself, or do I need to update my receiver? The only other thing I can think of is that I have not yet updated my player's (BDPS301) firmware since I bought it. I haven't had the player long, so I'm not too familiar yet with how firmware works. If anyone out there could please give me some advice I would really appreciate it. Thanks.