Unclear DTS dialog track

When I watched movies using the projector, I could not set the receiver in DTS or Dolby Digital mode, since I could hardly hear the dialog. I could only set the receiver in stereo mode.
Now I realize the center speaker carries the dialog. In my current setup, the center speaker is behind the screen, that may be the cause of this problem.
Update: I use the test tone of the receiver to check sound channels. There is no sound from the center speaker. I reconnect the wires for the center speaker and check again, test tone comes out of the center speaker. When I watch a movie with DTS audio, I can hear the dialog clearly.