When you plug your beaters make sure you can hear a “click” noise that assures they are plugged in all the way. If there is no noise when you stick the beaters all the way, the tabs could be broken. Examine the beaters for damage to the tabs, replace the beaters if necessary. If everything is good with the beaters, the mixer’s spring that holds the beaters could be damaged and it will need to be replaced.
Check out the section on beaters falling out on the KitchenAid KHM512ER Troubleshooting Page .