How to Troubleshoot and Fix a Faulty Mechanical Keyboard Key

1. Reconnect – Unplug and replug the keyboard (or try another USB port/cable). For wireless, switch to cable or replace the batteries.
2. Clean – Blow out debris with canned air; remove keycaps if needed.
3. Fix the switch – Remove the keycap and clean the switch with air or contact cleaner.
4. Check solder joints – Repair loose or broken solder points if switches are soldered.
5. Replace the switch – Swap it out (hot-swappable or soldered).
6. Last resort – Replace the entire keyboard if none of the above works.

Credit : https://www.lifewire.com/fix-mechanical-keyboard-keys-5209031