I just dealt with this yesterday. Log into your account on BA. Click on the top part where it says something like “ Mr/Mrs X” with a head symbol next to it. This should bring up a list of your flights you have in your account. You need to scroll down to it. Next to the flight on the right you will see “add frequent flyer number”. Click on that and add your AS ff number. You have to add it here even if you did it somewhere else before.