So...you have 2 AceKards and set them up the same. One works, one does not. Is this correct?
Assuming yes, try putting a couple folds of paper on the label side of the non-working card and see if it fixes the problem. If available, try it in another DS.
Also important, are you using a DSi? If so, you need the AceKard 2i version.
The above are hardware related tests, as it seems that your software is OK/not the problem. To test this, try swapping your uSD cards. If the non-working one now works, it's the software on the uSD.
Also, I highly suggest you use the AKAIO software instead of the official firmware. Check the wiki for reasons why (stickied on this board)