Finisher Effects doesn't display every time Rare Monster is killed

Finisher Effects doesn't show the effects everytime
Why does the Finisher Effects doesn't trigger every time a rare monster is killed? There are like 1-5 rare monsters in a map and it seems like working like 20% of the time. It seems it doesn't trigger if you kill the monster by your spell and not by the weapon. The item says "It amplifies the deaths of Rare Monsters you kill" but it felt more of a scam.