A reward code is not applying at checkout

Rewards become ordinary Shopify discount codes, which means Shopify, not the app, decides whether they can be used. The usual cause is Shopify's own Combines with setting: if the cart already has a sale price or another code and the reward is not set to combine with it, Shopify silently declines the reward. This is why a reward that works normally starts failing during a promotion. Check the discount's combination settings in Shopify before assuming the reward itself is broken.