If the knob is not worn and there is no set screw involved, the post may be split where the knob would cover it. Gently expand the post by using a small screwdriver in the split. Be careful, it doesn't take much to move it. That will keep the knob from falling off. If you can't get the knob back on, then get a cloth and a pair of duck bill pliers, wrap the post and again gently squeeze the post.