I tried to upgrade a Virgin Flexi flight with one of my complimentary upgrades on the East-West route.

I tried to upgrade a Virgin Flexi flight with one of my complimentary upgrades on the East-West route. I was told there was nothing available. I tried to upgrade the other leg (a saver fare) on points. Also without success. According to Virgin's website, this is subject to Reward Seat availability. But both flights show available reward seats. Is there another layer of discretion here?