Do they work if you run the cabling for your main fronts into them?

It's most likely a comms or config issue. The set up normally advocated by M is having the centre as Master so you have an M5 cable going from the G68 to the 5200C. Use the comms part of an S5 from the 5200C back to the 511 patch box. Use S5 cables for the front and rear pairs, with the comms lead which "drives" each pair connected to the 511.

The other issue is config. Have you set each speaker in the new pair to be a Rear Left and Rear Right?

Cheers

George