Unfortunately, scanning issues mostly occur due to older scanning technologies still used by some stores. Some devices (e.g. laser scanners or flat bed scanners) oftentimes have difficulties reading smartphone displays in general. We hope to be able to solve this problem soon though as most stores will hopefully replace these scanning devices with newer image-based scanners in the course of their next cash system update. Until then, you can always ask the cashier to type in the card number manually in these cases so that you don't miss out on any rewards points or discounts.