Sion, the Undead Juggernaut

I have been playing the new Sion and like him, but am having a serious problem using his abilities. At the beginning of a game (level 1- 7 or 8) they all work and then at a certain point Decimating Smash and Unstoppable Onslaught stop working completely. When I press the key for either of the two abilities I can even here the in-game noise when you press a binding to activate an ability. I don't know if it starts at a certain level or point in the match, but my bindings are correct. Glory in Death, Soul Furnace, and Roar of the Slayer work perfectly all the time. I think this has happened every match I've played with him (6 matches on Summoner's Rift).