You should be able to now.
After the giveaway ends, the key doesn't automatically get posted on your page. I have to manually press a "Send Key" button for it to show up on your page.
It's probably like this so that the sender is able to check and make sure that the winner is good before sending the key (no un-activated wins, no fake giveaways, no re-gifts of games they won, that kind of thing).