I've also noticed that it is somewhat inaccurate as of late. Sometimes even if I hit my target though and he is low in health he can somehow heal through it, almost as if it is having the opposite effect it is supposed to have.
This isn't an attempt to discredit your issue (which I know is super frustrating, having Ice Cometed many rats into Oblivion myself), but are you tab-targetting the person you're fighting? That has been the only way I've been able to get mine to reliably hit the target I'm after. Although these days, I'm noticing it targeting players/NPCs I have just finished killing, so I end up beaming their dead body instead of the new intended target, which is super unhelpful.