Try a different loader maybe? (I have really good luck with Wiiflow) You might also try changing the hook type. (Can't tell you exactly what one to use since I don't cheat at Mario Kart so I haven't had a chance to figure it out.)
It's just one of those things you have to play with to get working right.
Also since your using a loader, I would make sure your using a cheat from the right region for the right region of console and that the game is from the right region try to match everything up.... (Should all be US.) I am not sure but memory address's could be different in different regions and that could throw off the cheat? (Half awake here)
Last but not least, some cheats simply do not work. I have had it happen to me a few times, God bless them when they do and they let you skip all the tedious parts of the games!!! But it's pretty heart breaking when you realize that your going to need to spend 2,000 hours grinding some crap in a game to get that shiny orb!
Edit: One last thought, you might download Accio hacks and see if it has some different cheats. Maybe you have Mario Kart 1.01 and the normal auto cheats are downloading 1.0 or something? (I am guessing here)
