It might be also because you're using a larger sd card (sdxc). Use an sdhc model, one that's 32gb or below preferably..
Make sure data shows up on your pc, and the sd card adapter is in the unlocked position.
Try with a different sd card adapter if you have one.
You could try cleaning or blowing on the sd card reader on the wii, if there's any dust/debris stuck on there, visually inspect it with a flashlight and see if the pins are clean & visible.