Behead support doesn't work most of the time

Simple problem. Behead support just doesn't work, at least half of the time.

I have Behead support linked to Killing Palm, so there's no room for ambiguity here.

I cull every single rare I encounter, there's no other effects at play here (I have no Ignite or DoTs or anything of the sort).

Yet I only get a Behead buff around half of the time, or less.

Bump :)

Bump, dissapointing.

Behead II - if i get the buff it runs out after a few seconds(infight), tooltip says 20 seconds, there is something clearly wrong... it kinda looks like the granted buff only lasts for the next attack unlike the Headhunter buff, so maybe the tooltip is misleading for it's actual function.