Rear speakers do not work
Make sure of what audio mode you've set your receiver to, as well as what the source audio signal contains.
Some modes, such as Dolby Digital/Pro Logic will adopt the input format, and subsequently use only the front left and right speakers if there are no LFE/Center/Surround channels present. Setting the mode to Pro Logic II or IIx, on the other hand, will force a signal to be spread out to the various extra channels.
Using the Automatic Field Detect mode will usually provide the most appropriate solution for the input signal, otherwise.
Also note that if your audio source is from a PC and you are playing games, some games will, when you are using the digital output of many sound cards (most notably the Sound Blaster Live! and Audigy 1-2 and Audigy SE model cards) output only stereo as their signal, but still in a digital format. Your receiver will pick this up as a stereo specific signal and avoid using the subwoofer, center and surround channels.
Unfortunately, for those cards, there is no work-around beyond using analogue output or getting a more recent, better compatability sound card, as it is a limitation between the drivers, the card and the game in question, rather than your receiver that has the issue.
If the above doesn't help alleviate your issue and/or you could clarify on what your source audio is and whether you encounter this issue with all source-types or not, please post a follow up response.