Just go to the RJ or AY site and you should be able to select your seats without any issues. Check expertflyer before and after, just to make sure they’ve ’stuck’. Go to manage booking on either RJ or AY, enter the required details and then navigate to the seat selection section and make your choices but don’t forget to save. This has happened to me a lot with BA’s sausage roll (sorry, flakey IT) and this solution always works.