Sion's W passive not working.(Gameplay)

I just got finished with my first game as the new Sion. I was playing on Twisted Treeline. Early in the game I got my W, and I started killing everything, to try and get a lot of health. To my surprise when I looked at how much bonus health I was getting, it said zero. I had to have killed at least 20 minions since I leveled it. I also noticed that my max health was around where it should be for my level. I then went to lane and killed a couple minions and watched my health the whole time, never did it increase. I'm not really sure if there is anything one person could do to recreate the bug. I'm not sure if I did anything at all to create it. I also don't know if other's have had this problem, I might be the first. Very irritating bug to say the least, I was looking forward to being tanky, without building very many tank items.