Culling Strike not working.

I recently decided to try the Witch Hunter build, but I'm not noticing any changes with Culling Strike enabled. The game still requires mobs to be killed down to their last pixel of HP, instead of activating the Culling Strike.

Shouldn't it automatically work as soon as they reach the threshold?

Its a check on hit, so once they reach the threshold you have to hit them again for it to trigger.