If a coupon list runs out but the campaign is still live we will put those individuals in a 'waiting for coupon' status. We then retry coupon issue failures with an exponential backoff (i.e. with an increasing time between retries).
We perform 25 retries over approximately 21 days. Assuming the coupon list is re-filled within that time, coupons will get issued and the reward email will be sent.
After 25 times, we stop retrying, assuming that it will need manual intervention to issue a coupon.