Possible cause: If "only" a stereo signal is offered by a source, e.g. TV, the rear speakers remain mute despite 5.1. configuration, since stereo addresses only the front left/right channels.
Solution: If you want to play an incoming stereo signal on CINEBAR PRO AND EFFEKT speakers, select the Dolby Prologic II mode by pressing the "Surround" button on the remote control, which outputs the stereo signal on all channels.