Warwick W's passive is supposed to be global, and does not have a cooldown, as I understand correctly. I have noticed several times that, if I die and respawn again when an enemy is really low in health, the tracker/scent does not show up, it never notices the fact that the enemy is low. It is after the next champion gets low that the passive works fine again.
This seems to happen sporadically, but it's pretty anoying sometimes.
Can you take a look at it?