Custom potion effect with healing when effect expires (not working)

So i created a custom effect called "Ashen Nightstar". It is given by a custom bow. When an entity is hit by this custom bow, the user gets a stack of Ashen Nightstar buff. I want it to make it so when the effect ends, Ashen Nightstar will grant HP to its user based on the stack count. But somehow, it doesn't work, doesn't even add single HP. How can i fix it  though?