In general I've had great success with CSRs helping correct errors when things don't work properly on Delta.com (e.g., Matrix returns valid fare construction that must be manually entered).

However, the companion certificate is another ball of wax as its managed by the "Promotions Desk" and the Diamond Line reps can't help you with it. Basically, if the search tool on DL.com doesn't return a result nobody at the promotions desk will be able to help you.

I once spent over 3 hours on the phone going over each fare rule to prove my routing was a valid fare construction -- the agent, her supervisor, and "someone from IT" all agreed it should work but they were unable to issue it since the system rejected it (without explanation -- simply said "invalid").

The companion certificate is really disappointing -- I've forfeit it two out of three years I've had the platinum card.