I saw a thread on this for Cubase 6, but haven’t seen one since. I am using Cubase 8.5 and am having the same issue the users reported on Cubase 6.
If I use the Compressor that comes with Cubase and activate the Side Chain Input on the compressor, occasionally it will not show up on another channel’s send options. Some channels will recognize the side chain input, but others do not. Does anyone know what causes this and how it can be avoided?