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 :)