Asia miles tells you to contact Amex? I assume your booking was through Asia miles and you have enough Asia miles to redeem the flight?
if so, then Amex has nothing to do with this booking. Insist with Asia Miles that you want to issue the ticket for this award. Do not mention Amex points.
I think OP is saying that Asia Miles tells him to contact Cathay, and Cathay tells him to contact Asia Miles. OP did not mention Amex giving the runaround.