Another frequently overlooked issue with companion upgrades is that the companion must have their DL SkyMiles number in the reservation. If not, then you may not be added to the upgrade list.

The website claims that they allow upgrades for companions who are also partner elites, but I know I had problems with an AS MVP companion (when they were still partners) getting into C+ on my status so that might be a recent change:
http://www.delta.com/content/www/en_...-upgrades.html