I had the same problem a few weeks ago with my K3.
In my case a very small piece of plastic had come off the bottom of the SD card and had lodged itself under the pins at the bottom of the slot.
I was able to dislodge it by holding the camera with the SD slot facing down, and some gentle probing with a toothpick.
This might not be your issue, but try using a bright light to see if there's anything under the pins in the slot.
Good luck.
Fred.