Zend certified PHP/Magento developer

Magento 2 : Discount Codes not Working for Some Customers

we are using magento 2.4.5, we have created coupon codes and followed all the instructions – we are finding that the codes aren’t working for some customers but do work for most.

I have tested them on desktop, mobile, and incognito so I know the code works, but sometimes it doesn’t for customers, which is sporadic.
We have tried to see if it is a common browser or device theme and there isn’t.

Any pointers would be appreciated