This is likely just a setting in your theater or tv.
If you are not getting sound out of the center speaker then the theater or tv is set to just 2.0/2.1 mode.
If you were telling me you only get stereo sound out of all 5 speakers that could be the source device, but since you don't get anything from the other speakers then it has to be with the receiver itself.
Naturally do check the speaker connections on it.
EDITED:
If receiver is in direct/passthrough mode then it is because the source device is only outputting 2.0
If receiver is in dolby or some other mode and wont ouput to surround speakers it is an issue in the receiver