Just to build on that a bit, but if you have a complex set of flights, or you have amended the booking since the initial reservation, or AA has changed their flights (not much of an "if" about that) then it is possible you have one BA PNR and several AA locators. You need the right AA locator for AA's website to work and for seat selection purposes. You also need the reservation fully ticketed too, and even AA's system isn't immediately responsive in this area. So a combination of the AA locator and perhaps waiting a day or two should hopefully work for you. If you booked a simple there and back to JFK online and no changes then really the locator shown in BA.com should work, but it may be that you need to explicitly enter the correct name and locator into AA.com to get what you want.