Warwick's W (Blood Hunt) doesnt function properly

In the tooltip, it states that Warwick senses enemies below 50% globally and gains additional movement speed when moving towards them. And some of the time, this does happen. But sometimes, I will see an enemy below 50% health, somewhere on the map, yet the passive movespeed and sound effect will not trigger, even when moving towards them.

This usually tends to happen when the champion was below 50%, healed above 50%, and then was quickly brought back down below 50%. Or, in the most consistent case, I will have died as Warwick in a fight, then respawn with 1 or more enemies below 50%, or even lower. Yet Blood senses will not trigger,. In fact, the passive portion usually doesnt re-trigger on anyone until after you have activated W since you died.