Is it still under warranty?
If yes, STOP NOW! And see if you can return it under warranty for repair or replacement.
If it is not under warranty, then it could be a faulty switch, bad soldering, or even a broken trace on the PCB.
You'll need to open the keyboard (hence why it's better not to try this if it is still under warranty as opening the keyboard generally voids the warranty), and first of all try shorting the two pins on the D switch.
If it doesn't work, have a look and see if you can locate a broken trace around that switch. Have a look
here for what that looks like.
If that still does not work, try reflowing the solder around the pins on that switch.
If it still does not work, you might need to replace the switch. In the short term you could swap the switch with one of the 4 extras in the top right corner (assuming you have a full sized keyboard).