@rarther is right, with one more reason - offline terminals. This is particularly common with unmanned petrol stations, but I guess any shop can have connection problems and maybe then it’s trying to make offline transaction? Prepaid Monzo only allows for online only payments (with few exceptions like TfL and in-flight purchases, though last one is not always working).
Current Accounts and debit card will allow for offline purchases:
Edit: I got few more details from @RichardR, he explained that offline payments are still being tweaked and tested. Current CA cards that we have has better acceptance than the prepaid card but may still have issues.