Sion's W passive not working in the Jungle

While performing Sion jungle in a normal game, I noticed that when I killed the big monsters, such as Gromp, I would not receive the plus ten health to my W - Soul Furnace but when I would kill the smaller jungle monsters, such as the smaller wolves at the wolf camp. However, when I would kill the cannon minions in lane, I would receive the bonus ten health and I would receive the bonus two health from the melee and caster minions.