Support gem "Ambush" not working as intended on boss (unique monster)

Title.

I've experience quite often that for some reason my flicker strike didn't crit despite I supposedly having 100% crit rate against monster with full life. I made sure that the boss didn't take any any damage before I used flicker strike but I still didn't crit. I can see my Sniper's mark lingering where it should've been consumed by my flicker strike

Ambush gives "100% more crit chance", not 100% crit chance. So you could still have less then 100% crit chance. You can look ingame if you reach 100% crit chance on that skill vs full life enemies.

Though, I wonder if charged staff hits first, and then ambush doesnt count. Because I also do not get always crits on Xesht even though I have 100% crit chance with Ambush.

you need at least 50+% total critical chance first, then ambush gets you on 100+% against enemies on full life, becuase it doubles the current crit chance value (100% MORE)