'Target Champions Only' Not working for me.

Hello!

So, I've recently tried 'Target Champions Only', I put it on the toggle option, I set the toggle keybind to 'S' yet when I press it, it still seems to attack minions, I can't figure it out. So when I'm going for that juicy kill as Kalista in lane, instead of attacking the champ, it's attacking minions, (bare in mind I use attack move), it may only be 1 or 2 auto's before I have to manually correct it, but those 2 auto's could be a kill sometimes.

Any idea's why this is happening?