Good advice. Off the top of my head there seem to three obvious possibilities. First, the input on the ADAT is dead. Second, that particular channel on your ELCO has died. Third, something is happening before the signal is even getting to the ELCO cable (improper routing on the console that you thought was set properly, dead direct out etc...).
What I would do first is rule the source signal out. Run signal to your ADAT through channel three on your console or preamp. It probably will not work since this is the problem you seem to be describing. Now, on your console or preamp, switch from using ELCO channel 3 as a source cable to the ADAT, and run that signal down channel 2, or any other known working channel. If the signal shows up, you have just removed your source as the problem.
Next, take that same source and manually run a cable from it's output directly to the ADAT. This will mean that you are not using the ELCO cable. If the signal shows up in channel three, than your ELCO cable seems to be the culprit. If the signal does not show up, than it is likely that your ADAT needs repairs.
Next I would try