automatic talkback does not work

we do OVs for tv documentary. now we want to realise automatic talkback for the first time softwarebased with nuendo. manual says not a lot. i created a talkbackchannel in the controlroom, does not work. in preferences i choose "playback and record, thats what we need.
i`m not in the studio now, so my new idea is:
does it only work over a cue channel created? i think, lots of people need this function, so it must be me, who is missing something.
i appreciate any help, frank