aultvaich

I am new to DCC and am using an NCE PowerCab system. I am able to change CVs on my locos with no problems. I do have a number of coaches that have a function decoder fitted to operate the internal lights as well as tail lights but I don't seem to be able to find out how to set these up to work. Puttting a coach on the programming track I cannot get the PowerCab to talk to the decoder. What am I doing wrong? Any help would be appreciated. Many thanks