Thanks for taking the time to post in the Community and bringing this to my attention. It's my top priority to get this issue about collecting payments from customers using American Express straightened out right away.
I'd love to pull up your account and check why you're only having this problem with customers using American Express. However, I can't do this without asking for your account information. I value the security of your account, so I don't want your details posted here since this is a public forum.
I recommend contacting our Merchant Services Team. This team is able to securely gather sensitive information to pull up your account. Once they have this, they'll be able to further assist with determining the details of the payment and why it's not processing correctly.
Please know that I'm only a post away if you need additional assistance. I'm always here to provide help and answer any questions that you may have. Wishing you and your business all the best!