Zilean passive (5.21)

His passive is a little hit and miss. Sometimes it activates, sometimes it just walks over to the champ, sometimes it appears to do something, then stops without levelling the allied champ.

While it's possible that I'm not seeing bugs, and things are working properly, it's not at all clear to me why the particular behaviours are happening. Perhaps the champ levelled up while I was boosting him (this is the only thing I can think of for the random stops)? If so, I'd like a visual indicator to explain that happened.

Also, the cooldown feels a little long, as does the time I must be out of combat.