Sanc: Are those codes that never gave you the game last month codes you obtained from amazon prime directly, or codes you obtained from random "code giveaway" from the internet ?
If those are codes coming from giveaway threads over various internet places that give codes publicly, what can happen is that several people are entering the code but it happened during a "gog server hiccup" (very common when there are new games gog games given away by at Prime)
And when that happen instead of having the message "this code is in use/currently being redeemed" that tell you that the code have already been used by someone else, gog is bugging out and leave you enter the code as if it was you that was the 1st with the button to claim it, even tell you it will be added soon to your library, but in reality it goes nowhere because that code has already been redeemed by someone else before you, it's just that gog bugged out.
As otherwise if you were the fist, at most a game would take 24hours to appear in your library when gog servers are busy, not a whole month.
If you got those codes from amazon prime directly, that's an issue you should bring up to gog through their report system.
All direct from amazon prime gaming, last one that I added that hasnt shown up was added 4 days ago, the oldest is almost a month old. Cant resubmit the codes as theyre already used. Getting worried bc the oldest games I added are about to leave prime gaming and I still have not received the games yet