Welcome to Android Central! Did you make sure you pushed it in all the way? In many devices, the card will click into place. In others, you just have to make sure it's in all the way.
When you insert it, can you see an external SD card in the Settings>Storage menu? If not, remove the card and insert it into your computer (using a fullsize SD adapter, very cheap if you don't have one). If the computer doesn't recognize it, it's either faulty or unformatted. See if you can format it using the computer or the tablet.