Is it possible that, while the codes are not expired, your search terms are not consistent with those set forth in the coupon?

For instance, the coupon may say that it is for 20% off on "standard weekly or weekend rates" or something like that -- and you are searching for a three-day rental?

I have had that happen to me on more than one occasion. The other thing that I sometimes get is when I look to use the 300 bonus ZE1 point code for a rental smaller than full. That will show smaller cars as "rate not available" or something like that.