KitchenAid mixer bowls lock into place on the stand so that the bowl doesn't vibrate or become dislodged. The locking mechanism also allows use of the quick-dump method of cake mixing--where all the ingredients are dumped into the bowl at the same time (which can create strain on the machine)--rather than a traditional mixing method where the sugar, shortening and butter, for example, are creamed first. The bowl not locking can be rectified with some troubleshooting and cleaning steps.
Unplug the mixer and remove the bowl. Wash the bowl.
Remove the bowl support and locking mechanism. Unscrew the three screws located on the bottom of the mixer stand. Remove the vent screen and the mixer base. Remove the nut from the lifter rod inside the column, then remove the washer and spring. Remember the order---the washer goes between the spring and nut. Loosen the screw holding the bowl clip to the front of the bowl support.
Slide the bowl support down so that the tabs on the back of the bowl support line up with the slots in the column. Pass the tabs through the slots to remove the support. Wash the entire piece in soapy water. Use the toothbrush to remove any grunge. Rinse and dry.
Reverse the disassembly process to reassemble. But this time lock the bowl to the bowl support and locking mechanism by fitting the bowl supports over the locating pins and pressing down on the back of the bowl until the bowl pin snaps and locks into the spring latch. The locking mechanism should now work.
Hand wash the beaters, whips and dough hook in hot, sudsy water. Wipe the stand mixer with a clean, damp cloth. Don't use abrasives. Pay particular attention to crevices. Use a toothbrush to clean accumulated flour, dough and other grunge.