This is a recurring problem with donations over $5; I've even seen it happen with $10 donations. The first time I told the dev about this was probably some 4 years ago. Unfortunately the problem persists.
We Admins can create new codes, but there's no possible way to prove that you didn't get a code. The only time I've been able to help is if someone got a mail with a code that doesn't work, and that's not the problem I see the most.