(Too late for you, maybe, but not for others with same problems:)
I had the same malfunction: SD card didn't stayed lock in its slot.
After 2 weeks of considering whether to send it to japan or trying the local witchcraft,
I tried to push the SD in again,
while twisting a very little bit left, right, up, down, as if i were lock-picking
and suddenly - VOILA! it locked again!
Now the SD card slot is back to normal.
My assumption is, that somehow, by accident, i caused the hook to get stuck, but when i nudged it here and there, it got released.
Hope this tip will help someone :-)