Welcome to the forum!
Most (all?) displays (including this model) only send out 5.1 from the optical output on the display when the source is the TV's tuners. You should be running the optical (assuming the receiver does not have HDMI inputs) directly to the receiver from the other devices (i.e. Dish STB & BD player). This will get you the best sound, and avoid this issue completely.
Hope this helps!