I has the same issue earlier this month: TPA-LHR purchased through aa.com was 400$ more than the same flights, classes and fare rules on Expedia. I filled out the request for the Lowest Price Guarantee but got nothing more than the auto reply.
Was getting a bit antsy so called to cancel the reservation (the same day I made it) and re-purchased my flights on Expedia. I immediately got an email from an actual person saying that since I had chosen t cancel my flights they would stop looking into the Lowest Fare Gaurantee.
Made me loose faith in aa.com and especially their Lowest Price Guarantee. I did not care about the $50 voucher or whatever it is they give ou, but did not want to pay the extra $400.