Put knob back on. There is a clip that has to be slid into place after you put the knob in place.
The reason I went with AppliancePartsPros was that it was the easiest and clearest site to use. I was able to find the part I needed by its serial number (which was not true for some other sites) and it was clear from the site that it included the clip (which also was not true for some other sites).