My experience and belief is that they just lie. They award you some of the purchases to keep you on the hook but others they reject as 'not last click'.
I am at the moment still chasing some 5,600 avios that did not migrate from avios estore to BA estore - now they want me to spend an hour on the phone chasing it up.
I also have a further 8,000 which have not tracked through BA estore and just booke hotel.com for October for another 23,000 - which has not tracked.
Despite me using a dedicated browser, it's a lottery.