Square isn't going to contact you, but you can reach out to them. I'll post the info below on how to call or email.
Card declines can happen for 100s of reasons and 99.9% of the declines happen on the customer's credit card issuing bank. reasons like out of their buying pattern or out of their normal buying area, or too far away from the last physical payment location and lots of other reasons that a fraud department algorithm can decline a card, plus the other standard NSF or too close to limit etc. if it declines the first time it is going to decline the second and third time (unless ultra rare circumstance of a card misread). correlation does not equal causation, and in this case, they just happened to both be AMEX cards but we accepted over 50 Amex card payments this weekend so they do work.
You'll need your customer code to call. Follow the instructions below on how to get it.
https://squareup.com/help/contact
Click “More”
Click "I don't see my issue"
Click "Call Support"
Write down your Customer Code
Call 1 855 700 6000
Depending on your specific account, you might not have phone support available, or you might have limited hours of support. If your account doesn't have phone support, Email is the only way to communicate so that there is proper documentation of the conversations.
if you have a smartphone you can program the phone number with the code into your phone so it will be there next time, store it like this 1 855 700 6000,,xxxxxxxxxxxx. (your code goes where the xs are)(remove the spaces in the phone number). The commas tell the phone to wait 3 seconds each and will automatically enter it for you. Once you set this up one time it is there for all future needs and you won't have to go get your code again.
