Was it working before it was repaired?

Have you sent down a new config since you got it back?
If it's set for 8 Channels it could mess you up.

You know Center Channel has output on HD621 and you know the 568 Center works. Use that as a tool to narrow down where the Surrounds are failing.

So try swapping LFE/Center for the Surrounds on the Back of HD621.

If you get the Center Sound on the Surrounds then you know 568 is setup ok (for surrounds) and operating ok.
If center is now dead then that means nothing is coming out of HD621 for surrounds [on the channel you using anyway] (and your using what you know works on 568).

Keep in mind that surrounds might be rather quiet, but usually there is some ambience always present.

Don't move mains around for testing or that can really mess things up and confuse you. It sounds like those are working.

It could be the HD621 is defective. But you need to narrow down where the fault is.

- Fiddler (IMHO)

If you think listening to music is fun, try learning an instrument.
HT: G61RSL, 4x5K, 5.5KHC, 2xSC6000, Transporter, HD621, Philips BDP7501 4K, Tivo, 65" LG OLED 4K C6
Computer: Foobar2000, 5K, Mojo, HiFiMan HE1000 V2