In Game bug

Just finished up playing an ARAM as Singed, and during the game i notice my thornmail wasn't working against Jhin. I haven't had a problem with it on other champs i used it on in the last few game. So i don't know it this is a bug on Singed or a bug with Jhin vs Thornmail. Figure i would bring this up and see if maybe it was just a one time deal or something more.