This drove me crazy for a couple of days until I finally realized what's going on.
Cinput node has a bunch of invisible parameters whose value you can only drive with an exposed parameter plugged into the corresponding input.
Among those invisible parameters there's the fundamentally important “plane_index”, which tells the copInput node which plane to read the information from.

This said invisible parameter is not shown in the parameter tabs, and the copinput node will assume is “not active”.

“Not active” means that the node won't even assume a default 0 value.
It simply won't work unless you promote the parameter.

I wasn't aware of this gotcha, so I write it down for future users that might search for help in the future.

My question: is there any reason for the plane_index parameter not showing up in the parameter tab of CopInputNode the node?
Maybe something more general about the Vop Context that I'm not aware of?

cheers

If you leave it unconnected, it uses the plane index of the plane data that is currently being processed.