#1) They aren't on all the way yet.  It is better to turn the collar ring (spin) onto the silicone while you push it on, like you are screwing on a bottle cap than to just push it straight on.  It will go on all the way until it hits that largest bump (ridge) in the silicone.   Taking the collar ring off is easy, just reverse the direction of spin (like removing a cap).

#2) There is a film of oil (could be body oils, milk oils, moisturizers, etc.) on the surface of the outside of the flange where the collar ring goes.    Make up a batch of a paste made from baking soda and water.   Rub the paste on that area and let is completely dry.  Rinse with hot water.    The paste should remove the oil.   Use Dawn Dish detergent from now on.