1st thing to check, the physical connection - make sure the wires are good, maybe check them with the front channel and make sure they work well
2nd check your AVR and surround modes, if you have a 2-channel L/R source enable the virtual surround modes like dolby or DTS and see if they work or if you have a direct mode and a true surround source like a movie
more details always better, whats your setup details (make / model AVR, speakers, etc)