If you are trying to get 10A/B on a 747 that has swapped to a 777 (JFK-LHR) like me, the BA systems has 10B marked as a bassinett seat, so you cannot select this. That information is not marked anywhere so I could not work out why.

I had to phone BAEC gold line to get my seats sorted.

I had 10A but 12F assigned after the aircraft change.