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