Basically, it was the obverse design - there was far too much metal on the high profile first series George V portrait (compare it to the Edward VII larger head but much shallower relief); this caused not only weakness to parts of the reverse designs* but also the notorious 'ghosting' problem seen especially on pennies, halfpennies, florins, shillings and even sixpences. (*metal was 'sucked away' from the reverse) It drove the Mint demented trying to eliminate the problem which is why you see so many experiments going on: the 1911 hollow necks, the 1913 pennies, the 1915/16 recessed ear pennies, and the entire modification in 1920 that saw a shallower portrait. Of course, it wasn't finally sorted out until the Modified Effigy of 1925/26. But this is why so often Britannia has a weak face and chest, lion heads are weak, etc.