Key is not registering

First thing to check is to make sure the switch is fully seated into the socket. You'll feel it kind of pop and click into place. Try to pull it out with your fingers, if it comes out, it probably isn't seated all the way. It is advisable to support the PCB and sockets from the back side when installing switches so the flex of the PCB doesn't affect switches seating fully in the board, as demonstrated in the the how-to guide. If the switch is inserted fully, remove it with a switch puller and check to make sure the pins are straight. If they are bent, that is likely the reason it isn't registering. You can straighten them out and use them, or replace the switch. If you remove it, and the switch pins look good, take a look at the top of the socket. If you see any indications of scarring or indentation around the socket, the switch was most likely inserted incorrectly. The pin will catch the edge of the socket, and the pressure will cause the socket to lift off of the back of the PCB. If it is a popped socket the entire PCB might need replaced. Popped sockets are not covered under warranty due to the assembly required nature of the product.

If the key not registering is the function key (second key to the right of the spacebar), this key does not show up on the key tester layout in VIA. You can select "test matrix" to test this key. If the second LED on the right side of the board is lighting up, the key is operating correctly. You can also test it by pressing in the function key and pressing a numeric key at the same time. This will cause the function keys at the top to activate (pressing function and 1 will activate F1, et cetera).