Complementary upgrades require X, not C (at least I think it is X, but it definitely is different than C). Sometimes C becomes available first and sometimes X does. Presumably for your flight C is available and X is not.

Last week I was waiting for C on the mid-morning SFO-JFK (miles on a Y ticket) but X became available first and I got a free upgrade even though I was willing to pay miles. In fact, the C upgrade cleared as well about 30 minutes later and at one point I had three entries in my itinerary for the same flight – I called and they cleared it all up and I flew on a X upgrade.