According to my (impeccable) source, the kind of things that can affect reading of the chip are it being bent - if had been sat on - and a couple of other things, such as moisture getting in through a partial (but invisible) separation of the seal around the chip. When I said that the gates at LHR reject me more than 75% of the time, they were not suprised (they know LHR extraordinarily well). I said that I thought it would be quicker to go straight to the manual desk and they replied, that might well be the best thing to do - depending on the various queue lengths.
Mrs PAXboy goes through first time every time. The gates at LTN do not seem to be so picky. My passport even opened the gate in VCE last year!!