for some reason it only works on some 'is (DSi plural slang term xD), but try some different firmwares!
Hpscots0960, the _aio folder isn't where you out anything. Make any folder you want, you can name it NDS, DS, Games, DS Games or whatever you want. Put you DS Games in that folder, or just put the DS Game on the root of your card, that is, put the rom in the folder that _aio is in. The main folder of you microSD card.
So you could have a card with the following and it would work:
Root of card
++++++++++++- _aio
++++++++++++- GBA Games
++++++++++++- The Legend of Zelda The Minish Cap
++++++++++++- Mario vs. Donkey Kong
- DS Games -
++++++++++++Metroid Prime Hunters
++++++++++++New Super Mario Bros.
Here you would go into your MicroSD card when on your Acekard, go into DS Games, then press "A" on Metroid Prime Hunters to play it.