Scanning issues can sometimes occur due to older scanning technologies still being used by some stores. Some devices (e.g. laser scanners or flatbed scanners) have difficulties reading smartphone displays in general.
We are optimistic that this problem will soon be solved though as most stores will hopefully replace these older 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!
Modern image-based scanners can scan barcodes from smartphone displays within a few seconds.
Older laser scanners have difficulties scanning smartphone displays.