I have no idea what is going on. Coupons I won in the Canadian contest aren't working. I've tried different accounts, triple checked the book is couponable, used the coupon within 24 hours of winning it, etc. All I can think of is that kobo is giving out duplicate codes, and someone else used it before me. I have no way to verify this, of course.
Having said that, here are the codes I didn't use (and/or didn't work). I hope they work for someone. I didn't get much, with only the Canadian contest working.
30%
25%
20%
10%