suround sound 5.1

Hi I have a problem with my 5.1  on test f/r f/L C/ r/l r/r all work on test whe i put on sur sound on 5.1 the centre speaker only works on 2.1 the front 2 R and L work. its a ministry of sound MOSDV110 amd plugged into a 42in hitachi PLASMA any help appreciated.