Quick answer: If a code accepted but the balance looks wrong, you’re almost certainly looking at a different user account than the one you redeemed on. eShop funds attach to the exact account selected at redemption.
Your code went through, but the money isn't where you expected it. In nearly every case the funds are fine — they're just on a different account than the one you're looking at.
When you redeem a code, you first pick a user account, and the balance is added to that exact account. If your Switch has more than one profile (or you also have a separate US account), it's easy to redeem on one and then check another.
Switch to the account you redeemed on and look at its balance — here's how to check your eShop balance on a Switch or in a browser. If you find it on the wrong account, just sign in with that account to spend it.
If you redeemed a Nintendo Switch Online membership card, it adds membership time — not a dollar balance — so it won't appear as eShop funds at all. Our eShop card vs Switch Online guide explains the difference.
If you've checked every account and the funds genuinely aren't there, reach out with your order number and we'll help track them down.