Irelia Q sometimes doesn't 1 shot caster minions

so basically sometimes I can't 1 shot caster minions even after having blade of the ruined king. For example, on my last match, I had 3 items: Bork, wits end, divine sunderer but I still sometimes couldn't kill caster minions. Why can I sometimes kill them but sometimes can't? I was lvl 16 as well. Its really hard to play with these inconsistencies, is this only happening to me?