Cho'Gath Ultimate not giving stacks

I was playing an ARAM as Cho'Gath on the top purple team. Once I hit level 6 I upgraded my ultimate to level 1. I used my ult on a normal melee minion, and killed it. Upon killing it I did not receive the stack for the ultimate. I didn't get the health bonus, physical size bonus, or the icon in my HUD above the spells (where stacks, buffs and debuffs appear). I tried this again 2-3 times with minions and still nothing. Later in the game I used the ultimate to get a kill on a champion, and once again received no reward. I'm not sure what items I had when I hit level 6, but I bought a then . The next items I got were and. I've included my dxdiag, and the game log file from the date when it happened. Also, here is the link for the Match history. I've tried to include everything I could think of. Let me know if there is any more info I can give you, I'd be more then happy to help.