Sounds like either...
1. Your tablet can't handle a card of that size (not physical size but amount of storage). Most devices have a limit to what they can access.
2. The port has dirt, dust, or other debris in it that needs to be removed so the card can be read.
3. There is something bent in the port that needs to be careful straightened so the card can be accessed.
4. There is something actually wrong with the port and it needs to be repaired or replaced.