Serpent Sting Deathbox Not Working

Howdy howdy,
Ever since I got the Serpent Sting & its corresponding Deathbox a couple months back, I've noticed that most of the time the Deathbox simply doesn't work. I've made sure to manually equip the box and save & exit, yet when I make kills in-game it's still the normal deathbox. I really have no idea why this is happening, as my friend with the deathbox from the Final Fantasy event has never had this problem. I've also attached some screenshots to show that I both do have it equipped and that I own it in the first place.