Single Key Not Working
If a single key is not working on your keyboard, it's likely a hardware issue. There are three main things to check, in order of likelihood.
Check the switch
If you're using a hotswap keyboard, it's often that you'll install the switch slightly off-center and cause a pin to bend instead of going into the socket. Try removing the switch and checking if the pins are bent. If they are, you can use a pair of tweezers to bend them back into place.
If the switch was not bent, then the switch itself may be faulty. Try replacing the switch with a known working switch and see if that fixes the issue.
Check the hotswap socket
If you're using a hotswap keyboard, sometimes the pressure contacts on the back are bent and not making contact with the switch. Take your plate or case off and try bending the gold contacts on the back of the socket so that they make contact with the switch.
Another rare possibility is that the hotswap socket wasn't soldered down properly. Try giving a bit of extra solder to each side of the socket to ensure it's making contact with the PCB.
Check the diode
As a last resort, check to make sure the diode is in the same direction as all the other, soldered down properly, and not cracked. The diode is very the small rectangle with silver solder on each end. There's one for each switch, and the one closest to the switch is the one you're looking for. Try reheating or adding some solder to each side of the diode.
Still not working?
The above troubleshooting steps have solved every single issue we've run into for single key failures, but if you've tried all of the above and the key still isn't working, reach out to us on Discord for help!