Graves's autos sometimes dont function

Time and again, I've seen and experienced an instance of where graves will try to auto attack someone. The sound of him shooting happens, he makes the attack animation, but nothing happens. It still shows he has 1 round left but has to stop to reload even though his did not actually "attack". This seems to happen most often with his second auto attack, but is has occurred on the first auto attack sometimes too. If this is supposed to happen, i would like to know why. And before you ask, no i was not lagging.