EDIT : nevermind, it's probably because I'm attacking an opponent of the wrong type. ^^

(Not sure of the names : Charm is the red epic female demon, her third ability is a single target damage spell that has 30% increased crit chance)

I have 87% crit chance on Charm. Sometimes (rarely, just happened in Arena), my damage single target spell doesn't crit even though it should have 117% crit chance.