for a 5.1 system (sounds like you have a 5.1 system) your surrounds should be to the side of your listening area, and plugged unto the L/R surround outputs. The reason you are not getting any content from your "rear" surrounds is because there are not alot of dvd's that actually use the 6th channel (for mono 7.1) in surrounds.
This diagram should give you an idea of how your setup should be (if you can do it like this, it would be ideal)
Now, you can have your surrounds a bit further back from your listening position (mabey 2-3 feet) depending on your preference. But they should be to the SIDES of your listening position if you can.
But to answer your question, you have them plugged into the wrong jacks, they should be plugged into the L/R surround outputs.
