make sure you are using the right version of the rom... some codes only work for certain versions (such as mario rpg 1.0 instead of 1.1, or the european version and not the american)
Some codes may only work at specific times, as well.. for instance if you don't have geno, using the geno codes might just burk it all up.
I think I'm going to second Deathlike2 on this one. I mean, it's courteous to be clear. Consider it good etiquette.
If you're having problems with codes, it's generally a good idea to post them here and let us have a look at them. But please take time to press the proper buttons on your keyboard to double-check your post for clarity and accuracy. Use the EDIT POST button if you need to.
Why don't you just play like a real gamer and not put cheat codes in at all. No offense mind you, but I mean seriously? But ANYONE who uses cheat codes to gain an unnatural advantage in any game is no gamer in my book. That's what me and my friend like to call fake gamers. Also can you please write like someone with some common sense too? Thank you. I've never used cheat codes for any SNES game that I beat and refuse to ever use gamegenie/gameshark. What kind of cheat codes do you use in Mario RPG anyways and I can evaluate your noobness to proness level.