What is the likely cause if the beater rotates but strikes the side of the bowl?

If the beater hits the sides of the bowl, the bowl retaining pins are likely worn, causing the bowl to move on the pins. To fix this, the old pins should be removed by twisting and pulling upward, and new pins installed by placing them in the holes, supporting the arm ends with a heavy object, and driving the pins down gently. (Page 28)