White Nail does not trigger when targets hit with precision hits are also affected by Hive Disrupter.
[b]Destiny Hot Fix - 01/12/2015[/b]
[quote]On the Sniper Rifle, Black Hammer, the Hive Breaker upgrade has been disabled until it is fixed to play nicely with the White Nail upgrade
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By "disabled" Bungie hid the Hive Disruptor perk on the Black Hammer. It's still there, but not unlockable, thus making it impossible to give your Black Hammer the yellow border that signified a fully upgraded status.
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Thank you for reporting this. It is a known issue that there a few cases where White Nail will not trigger correctly. The team is aware and working on the issue.