Some corrosion might be built up on the SD card slot contacts. Find a card, and insert and reinsert it at least 40-50 times, then use a can of air to clean out the port. When using a can of air, fire in short bursts, and let the can warm up so it performs optimally.
If that still doesn't fix the problem, a pin might be bent inside the socket, which is difficult to repair due to the nature of the socket.