The main cause for the majority of the holes in German bombers was simple ball ammo. RAF fighters usually had 3 or 4 guns with ball ammo 1 or 2 with AP.
The problem, as someone already said, is that the bullets would deflect or start to tumble and not hit at the correct angle. Only a quarter of the armour piercing rounds fired into the Blenheim reached the armoured plate and hardly any penetrated it.
The same round fired straight at a 12mm armoured plate (8mm thicker than the Blenheims) from the same distance (180m) penetrated the armour 100% of the time. Airframe makes a huge difference.
The Germans knew that AP rounds were ineffective against bombers and they eventually removed them from the ammo belts, except for specialised missions.
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