Why the Inner Sight almost not working on bosses?

Is the Inner Sight’s gauge count from the percentage of damage that done to enemy’s maximum health? Cuz I can’t figured out any other way to explain why it only get 1% of the gauge per hit from hitting bosses.

My friend suddenly has all his arrows doing next to no damage on pit bosses and only pit bosses. He was burning bosses like crazy then upgraded a ring and now can’t do any damage.