Did you do this? Sounds like you didn't use the PRG to make the purchase, how/why do you think Amex would know? The credits are automatic, in most cases.
Consumer $100 Airline Fee Credit
Benefit is available to Premier Rewards Gold Card Members only. To receive statement credits of up to $100 per calendar year toward incidental air travel fees, Card Member must select a qualifying airline at
http://www.americanexpress.com/selectafc.
Statement Credits: Incidental air travel fees must be charged to the Card Member on the eligible Card Account for the benefit to apply. Purchases made by both the Basic and Additional Card Members on the eligible Card Account are eligible for statement credits. However, each Card Account is eligible for up to a total of $100 per calendar year in statement credits across all Cards on the Account. Incidental air travel fees must be separate charges from airline ticket charges. Fees not charged by the Card Member's airline of choice (e.g. wireless internet and fees incurred with airline alliance partners) do not qualify for statement credits. Incidental air travel fees charged prior to selection of a qualifying airline are not eligible for statement credits.