A while back I called in because cert-based search wasn't returning results that even the DL site said were available when searching for 2 pax without the cert. The agent immediately confirmed the same availability I was seeing, which was a good sign, but was puzzled that they couldn't get it to book either. "Let me try this another way..." nope, that didn't work either. Put me on hold for a few minutes, then came back and said "ok you're all good to go." As this agent seemed like the outgoing/chatty sort (you know how you can kinda tell) I asked what the magic sauce was, and they told me that although I and Z were clearly available, when applying the companion cert the computer wanted to force it into a higher fare bucket for no apparent reason. The way they got it to work was to go into Delta Term "which is like an older DOS interface" and force it to accept I/Z and then it was able to be ticketed.

The challenge now I guess is how to reliably get an agent who knows how to use the CLI interface...