I am new to home audio and have a small problem with a very simple set-up. Basically, I have 4 speakers hooked up to my Yamaha receiver, 2 plugged in for 'front', 1 for 'center' and 2 for 'rear.' My problem is, when I test the channels (a feature on the receiver where the channel is displayed and static comes from the speaker) the rear speakers make no sound when 'Rear' gets tested. The front and center work fine. I know the wire and speakers work fine from testing them on the 'front' input. Any suggestions??