Does the beater need to be cleaned?
If your beater does not fit on the beater shaft, food particles may have become lodged in the opening on the beater. Wash and dry beaters immediately, and do not let them soak in the sink with food particles. Use a toothpick to remove any food particles stuck in the opening gently. Do not store the beater on the mixer.
Is the beater appropriately aligned?
Align the "keyhole" opening on the bottom of the mixer, with the beater shaft and pin and twist in place to attach. It is not necessary to depress the "pin" on the beater shaft to connect the beater. The pin is stationary.