If you're getting stereo from a mono plug, the digital connection is working just fine.
The thing is that you can only get 2 channel PCM or 5.1 using dolby digital or DTS. Almost nothing will encode 5.1 to either of those formats (other than a hardware encoder on game consoles or a special driver for some sound cards).
Check your player and make sure the output is set to bitstream and not PCM - this will enable bitstreaming output which will let you get dolby digital or DTS out the port. Then to get it you need to have content with Dolby digital or DTS audio. If you don't,the best you get will be stereo.