Pit Boss Stagger Issue

Why is the pit boss close to unkillable (remaining 5-10% of health) unless stagger is activated? Stagger is in a bad state right now, because sometimes it just stops working all together and the cast damage applied to the pit boss from my character stops as well.

I don’t know if/what was changed about stagger, but something is off. It wasn’t doing this a few months back and the pit boss was ok to complete and advance to the next level.

I clear normal mobs/elites just fine, it just when it get to the pit boss, its a problem. Thanks for reading.