First, did you verify the AMEX credit was in your UberCash before the ride? If not, look now to see if it is still there. What form of payment did you use for the ride? Look at the bottom of your receipt. AMEX credit will show as UberCash and often I get a graphic notifying me my AMEX credit was applied.

From your emailed receipt you can usually switch payment method, for 24 hours after the ride I think. If AMEX credit is still in UberCash, then just try to switch payment method now (after the ride).