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I've had that happen, and mods told me to still wait a week since winning, then another week since first contact, dunno if it was implied by your comment, but there you go

There is more to this too. If you cant get a hold of the winner through SG, then messaging on Steam is often our go-to plan B - even advised by support.
But that can lead to issues with valve/steam for the one doing the messaging, as steam has issued suspensions, trade bans, etc on people doing so in the past. Any mention of Steamgifts, or even a phrase like "you have won" can be picked up upon and judged as soliciting, fake trading, or otherwise scammy behaviour - especially when sent from an account not normally associated with winners account (i.e., not previously friends or family member).
So be careful what you write on winners steam account message board.