I have written before about problems securing TSA Precheck when travelling on USAirways, even on the same ticket as an American flight when you secure Precheck.
I always credit my AA/US flight to my BA account so it’s not that which is the problem.
I have found that by looking at your UK site booked ticket on the AA US web site, you can enter your Known Traveller ID from your Global Entry record. This does not work on americanairlines.co.uk but you can book there and switch to the US site and pull up the ticket.
This doesn’t copy through to your US flights unfortunately.
A couple of months ago I called US and found that they can manually add your date of birth and Known Traveller ID manually. Hey Presto – I got PreCheck on that flight.
So, call if you want to add your ID, at least for now.