Exploit weakness (talent) behaves erraticaly

[synek]

Hi, I am playing brawler in 1.4.9

Exploit weakness is working wierd - i have it on lvl 5.
- it activates after large number of attacks/hits on target (over 15 hits - 8 attacks) to invoke some (eg. 5%) phys res reduction
- it does not work on some monsters at all (or you just kill them faster than it invokes)

Does it have something like internal cooldown to invoke (as it is sustain it should be on all time) or it checks some saves? If not it is probably broken.