Witchhunter Culling strike not triggering

I've noticed over the past 2 days after I started using the culling strike ascendacy node, It doesn't always seem to trigger

I noticed this mainly with Yama the white, He was below the culling strike threshold, I believe I've increased mine to 7%, He was less then 5% and nothing was triggering culling strike

I was shooting him with armour piercing rounds, spamming grenades, thought possibly it could have been do to an energy shield? but I was able to finish him off from 5% to 0% with armour piercing rounds but culling strike never triggered

Its worked on other bosses, other enemies, but has anybody else experienced this?

Is this a bug?