Hi,
To allow the winner to upload the files you have to formally choose the winner of the contest.
I don't know contest page from Contest Holder perspective but there have to be some
kind of "choose the winner" button.
You don't have to give winner all first places. If you want to get more versions - just specify them.
Let the other designers also get their credits (and points).
If you still can't choose the winner, there can be some error, so please contact support.