Is the bowl installed correctly?
It is important to push down on the back of the bowl until you hear an audible "snap". If you do not hear the snap, the bowl may appear level, but it will not be locked into place, and the beater will hit the sides of the bowl.
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Do you have a glass bowl?
It can sometimes be difficult to snap the bowl into place on new glass bowl models. This will become easier with time. Push firmly down on the back of the bowl with two hands until you hear it "snap" into place.
Are the clips worn?
Over time, it is possible that the bowl clip can become worn and will no longer hold the bowl in place. If this should occur, replacement bowl clips are available.