Ok, before switching SD cards to best to unmount them. Go into the settings>storage>unmount SD card. Once that is done insert the new card and the phone should pick it up and ask to format it for you. If not the other thing to do first is to format it via your computer if you can and make sure its formatted as FAT32, you'll see the format options beforehand. I take it when you've bought a SD card you're basically gone for a larger capacity SD card? If so you'll need to copy everything over from your previous SD card to your new one. Again this can be done by copying all your files to your computer and then copying back over to your new card. This will make sure all your data and apps still work. Afterwards if it still doesn't find your new SD card then it maybe a make it just doesn't except. I take it the SD card that came with the phone it picked up ok?