Every thread is saying that the app should automatically recognize AMEX. The card number is in the format: XXXX XXXXXX XXXXX which is 15 digits; yet, it cuts off at the 13th digit.
How do I fix this problem?
It sounds like the card number your customer provided you does not match a valid AMEX. On your app there is a little card where you enter the credit card number. This little image will display the card issuer after entering two digits. On your app does it display the miniature AMEX card?