This happens if you are not selecting a player when your using it(no unit). Much like refilling a bottle of water you have to find the 'sweet spot' on the unconscious, which is hard since if you walk into the body you push it away.

However I can say I have applied a defibrillator correctly to a person, where it came up with a message with the specific persons name( I don't know if he woke up as he logged out shortly after).