Damage to mini-bosses does not pass. (Deep Rock Galactic)

If the boss's health bar is "gray" - it means he is invulnerable at this stage, you need to either eliminate the enemies, or hammer them according to the definition. points.
The tyrant "closes" in a cocoon and does not open until the spit-spine / regenerating flower is blown apart, after which the core is vulnerable.
Likewise, with bosses - either his "minions" are still alive (Guardian of the Hive), or you need to shoot at his processes before the vulnerable ovipositor opens; either he is increasing his armor (Dreadnought); Gemini are invulnerable if one heals the other etc.