Gifts Not Marked Received

Usually dropping them a comment or adding them on Steam to remind them have done the trick for me.

I usually assume they can't log into Steam client at the moment to activate the key.

1. Wait.
2. About 5 days after giveaway ends, I post a comment on one of their giveaway or on steam profile or add them on steam.
3. After 7 days run crying to support (didn't happen yet for people who were online but didn't redeem)

I send an email to their address OR drop a Steam community profile comment.

If still no answer by the time the seven day passes, I check them again for infractions.
If they have a history of unactivated keys, I ask for a forced received feedback, showing the communication attempts and the revealed key (obscured a little, of course).
If their profile is clean, I ask for a reroll.
If it is their first win, well… then I check the price of the game and gamble: if it is some cheap junk, I ask for a reroll, if not, then I send a received feedback ticket and pray support can either make contact with the guy or approve it.