Typically, it is the switch itself. Some boards support switch disassembly without without de-soldering (likely not true for kinesis advantage2). If it is supported then pulling the top switch cover and cleaning it may help. Otherwise you need to de-solder it all and replace the switch.
You can check the switch with ohm-meter on powered off keyboard. Measure continuity on the switch while it is pressed/released (measure against the corresponding diode direction). It should give (near) disconnected result while the switch is not pressed. If it does not give a clear connected/short result when the switch is pressed then it is definitely bad. Otherwise you do not really know and you need to check with oscilloscope on a live (powered and functioning) keyboard.
