Are you sure? Going through my combat logs says that artic blast is procing Hrothgar
On NPCs is sometimes weird. I have almost never visual, sound and so on if I proc Hrothgar on them, but in Combat Metrics it appears. In case of Arctic Blast its propably because that skill itself already has a very dominant visual, maybe it just overshadows the Hrothgar one.
Was testing it for an hour, because recently spent 500 000 on it..All other stuns proc Hrothgars chill except this one.. What a shame..