I was going to post something fairly similar (well, minus the champers obvs). I was thinking on my last 380 flight, which is the longish daytime service to Vancouver, that BA had somehow improved the latch, since it was working better, but in fact all that has happened is that I've ingrained the knack to it. My version is that it is pressure and timing dependent so you can either push the side of the latch as above or introduce a pregnant pause into the process and do a slight sideways movement with the palm of your hand or thumb. Doesn't help with the middle distance thump thump thump however, it appear some people only fly once or twice a year, strangely.