Inside the knob shaft there is a clip that lines the inside of the shaft. Typically this clip provides a more secure fit. However, some models require removal of this clip in order for the knob to fit. To do so you will need a small flathead screwdriver.

Looking at the clip inside the shaft, you will notice some slits or notches in the clip. Insert the flathead end of the screwdriver into one of these notches and apply some leverage to wedge the clip out of the shaft. For a visual reference see the video below. 

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