Listenback / Talkback problem

[Mano]

I can't seem to get listenback input. I did everything the manuals say, but it won't work. Is the wiring also wrong like it was with older main-units ?
I tried connecting several dynamic and condenser mikes but still no result.
Also the parameters in the main-unit utility-pages seem to be in order, what am I missing here???
The pots on the backside make no difference if I turn them up or down.
Talkback works and slate works , why not
the listenback???