I've had this happen on several occasions over the past two years. All AC classes of service qualify for MP credit, except of course for award classes. However, AC's TANGO class fares only receive 50% credit. On one occasion, it took three letters with copies of eticket, boarding pass and MP web site print out showing eligible AC fare categories. On another, even though I was entitled to 100% mileage (on an AC TANGO+ fare) I only received 50% credit. Gave up after three letters to MP (only another 250 miles). It can be an agonizing process...