So many possibilities:
- faulty wiring
- surround speakers not activated (stereo only)
- defect in the amplifier stages
- surround speakers defect
- source of audio doesn't provide surround channels
- ...
Please describe your situation more detailed.
Has it worked before and stopped working or has it never worked?
If it worked before, what happened/what did you do prior to the fault?
What have you done so far to find the problem?