The computer system tells the gate staff which bags have been "seen" and "authorised" for loading on to the flight. It does not tell them if a bag has made it in reality.

There are various reasons why bags fail to make the flight and, thanks to historical events, we are not allowed to ship them onwards until after you have arrived at your destination and reported your bag(s) as missing. Once they are reported, loading them as "Rush" on another flight and repatriating them is quick.

Frustrating as it may be, reliability of passenger baggage loading is a whole lot better than it was when I first joined BA.