Cannot target minions in Summoners rift

So i was playing Darius in a summoners rift game and everything was fine... But occasionally i would try to target minions and the red sword should appear right... well it didn't and i would end up aimlessly moving around the minions. I would tab out and back in and it would fix it... but then soon it would happen again..I dunno if this is a glitch for just Darius or if other players have experienced this but can you please look into it? Thanks