Did you read the accrual rules on AA.com?
http://www.aa.com/aa/i18nForward.do?...ishAirways.jsp
Earning AAdvantage Miles
Earn a minimum of 500 AAdvantageŽ miles each time you purchase and fly on a published eligible fare ticket on British Airways, except for miles earned on eligible Discount Economy Class Fares, which will be 25% of the normal minimum (125 AAdvantage miles).
Let me guess what kind of ticket you had...maybe, "Discount Economy" (which, on BA, is anything in coach except Y/B/H)?