The DSi's detection of the Acekard has nothing to do with the MicroSD.
Therefore, there is an issue with the Acekard itself. It's probably the ubiquitous contact issue, and you'll just need to use the paper trick.
That just means to shove a piece of paper between the cart's contacts and its shell. If that's too hard, you can also shove it between the cart slot and the cart itself.