Tibbers Bugged
I've been experiencing this for the longest time, and I just assumed it was a temporary thing. When playing Annie and commanding Tibbers to attack an inhibitor, he will just stand at the border of said inhibitor and do nothing. He will only attack it if he just got through killing a nearby minion/champ and there is nothing else to aggro onto AND only if you make no commands after. This is something that should be fixed soonish if possible.
Sidenote: also for some reason I am unable to submit this as a ticket so there's that.