HI,
Put the channel strip on an extender and probe the COMPUTER SEND from the VCA card. This is where the VCA voltage is sent to the computer and looped back via the COMPUTER RETURN. Each bucket (8 channels) have its own S29 cable that's connected in the computer front panel. You can find the S29 pinout on the computer/console service manual under the cabling section.
Test first the channel strip to make sure you have the analog signal on the COMPUTER SEND. If you don't, that means you have a faulty component/loose cable somewhere on the channel, if other than that everything is OK, I would bet on VCA card.
If you have the correct voltage, then you can check where the signal is entering if you go the the computer !TEST menu and go to the Analog Input test. You can move the fader and check 1) there's any value moving 2) if it is, if it's mapped onto the correct computer channel.
With this you can easily pinpoint if it is a hardware failure, where it is located roughly, or software misconfiguration.
PS: regarding the TR card: the COMPUTER SEND is used both for the VCA voltage or the TR voltage of the selected object. This switch is made when you shift between TR mode and the VCA mode. This switch i think happens on the VCA card for VCA consoles (I have ultimation).
Good luck!
Artur
