Make sure you DVD is set to DD5.1 in the audio menu. It may be set to PCM or Stereo which would exhibit the symptom you are observing. 3-channel stereo is a DSP function, ie the digital signal processor is processing the signal and creating a center image. Dolby Digital takes the 5.1 channels of information directly from the source. If the source is stereo (DVD set to PCM or stereo), your receiver is likely autosensing that and is only sending the signal to the L&R speakers.
Enjoy,
TIC
Enjoy,
TIC