Why are some keys not working?

Keyboard keys are not immune from failure. I just replaced a KBD because of a few dead keys.

It is also possible one PCB trace could open, disabling a few keys. I had that happen on a KBD that wine spilled on and eat the copper trace.

It may be possible to access individual keys on some KBDs and clean them, fix the spring mechanism, or replace the switch.

Otherwise you must replace the whole KBD.