Experiencing issues with Shop Pay, what could be the problem?

[DavidPoenaru]

Hello! 
I have some trouble with Shop Pay.
A friend of mine purchased an item from my store and it worked like a charm, however, another friend tried to purchase, and an error occurred. It charged him for the product but afterwards, a message showed up saying "There was an issue processing your payment. Try again or use a different payment method."
This is a very big problem because he got charged, but the order was not processed and he did not receive any mail, nor did I receive an order notification.
Please respond, I want to start advertising but this makes my store unfunctional.
Regards.