A lot of people are reporting that their Nokia Lumia can’t read their micro sd card. Here are steps you can take to fix this issue.
Try to re-seat it firmly and clicked down.
If it’s a used SD Card, try formatting (FAT32) on a computer and try installing them again.
You could try resetting it to factory default settings (Settings>About>Reset Your Phone) and seeing if that helps. You will need to setup the phone again from scratch, but it might be worth a shot to see if it fixes the issue.
If you exhaust above, try reporting this issue to the vendor and you will most likely get a replacement phone as the phone could be defective. Or consider buying a Sandisk Ultra which is guaranteed to work with the Lumia Windows 8 Phones.