Wyvern Coated Knives Not Activating

fns2005

I've recently started using the Wyvern Coated Knives and have noticed sometimes it doesn't fire off. It is not because it misses or the enemy has already died. I'm very close to them and the enemy clearly still has health. I have to make sure it cast or cast it again. Also it seems you have to be right up in the enemy's face for the artifact to work. Not sure if it's always had such a low range or I just can't see how far it reaches. Some clearer visuals of what the knives hit would help.