The sub will not work with stereo mode if the mains are set to large, and the sub will not be noticeable if the mains are small but have a really low crossover point unless the material being played has low frequency material. First step, therefore, is to set the mains to "small". You may still see very little sub activity in stereo mode if the crossover point for your mains is low (say 60Hz or lower), since many CD's contain very little information below 50Hz.