I've had the same issue - when I escalated to Amex they told me the Uber cash balance had to be used up and then the Amex credits would apply - this never used to be the case (must be a recent app update) and i felt would be outrageous if you had hundreds of dollars in Uber cash - I only have $30 so will use that up but still think it's not on and if anyone has any tips / workarounds much appreciated