Since 1.1.2. the white nail perk doesn't always trigger after landing 3 precision hits. Did sepik's prime today and had to reload 3 times in a row after 3 precision shots. I have never seen this glitch/bug before 1.1.2.
Also, the immune target thing is still not fixed since the white nail perk still triggers when landing hits on the Devil Walker's legs, just saying.
HOPE it get fixed with the return of the hive disruptor perk in HoW.
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From my experience, if a boss is suffering lingering damage (I.e. Thorn burn or grenade damage such as solar or even the Titan lightning grenade) the white nail perk will not activate. So that might be what happened to you if you were causing lingering damage or sustaining damage. Just my 2 cents.