I was doing pretty bad with that actually...I could not possibly tell you why, but I kept confusing when I actually had a charge to use. I have it all squared away now. I saw "1" on my little icon and I would say, "Oh. I have a charge" and then kill a minion and get nothing. I don't know why it took me so long to put together what was going on---I think I was just focused too much on everything else. I quickly became afraid of angering my bot buddy by screwing up when I had a charge ready or not that I hardly touched minions and used my charges. It seems silly and dumb, but I blame it on new champ and new role. I'm about to go into a 4th game. We'll see how it goes.
Remember that your flay passive gives you the enhanced AA, so it does a bit more damage than you think it will.