Questions on Assassin's Focus stacks

[Prof Evbof]

I am playing a Stalker - Spines/Bio Armor - and so far have only taken Lunge and Assassin's Impale from the Spines set.

Sometimes when my Lunge hits, I do not get a stack of Assassin's Focus. Aren't I supposed to automatically get a stack or is there a proc chance that I am unaware of?

I have been monitoring my combat log because of this.

On one occasion, I looked back in my log and could find an instance where I got three stacks of Assassin's Focus, but my next Assassin's Impale did not crit. Do stacks of Assassin's Focus go away on their own over time or did the rng roll something over 99% or is there another thing going on that I am unaware of?