To resolve this issue, you must contact GOG Support and ask that they reset your code. The Backer who owns the code can then redeem the remainder of the games that were not selected when the code was first redeemed. 

Note: Redeeming the code by selecting all the games (instead of only one or a few) simply places the games in the GOG library of your account - it will not download or install the games unless your account settings have this option enabled.