I did see this blurb in the T&C's:

Taxes and fees are subject to change. Reservations are only available at delta.com. Companion certificate terms and conditions may vary each year of Cardmembership. Additional restrictions apply.

Not sure how the OP called in on his cert considering they're supposedly only redeemable online. I tried a couple of sample searches using the cert for LAS-SEA r/t in July and August. All came back with the search 'error'. I checked as if I were paying and there are a ton of L and T fares for the dates in question. Not sure why the search using the cert isn't coming back with anything? Tried some other destinations originating from LAS....same problem...hoping it's just a glitchy search engine tonight.