Lux's E

Everything was fine until about mid game, so maybe 15 minutes into the game. I noticed her e was not casting. I tried and tried to cast it, but all it kept saying was "you cant cast this right now" as if I wasnt maxing it out and it had three levels on it already. It went on like this throughout the whole game after. Not sure what it is, but its very inconvenient. I havent tried to enter another game with her since because I'm afraid of it happening again.