Fill the bottle completely with very little air space at the top. Put on the cap, turn it spout down and smack it on the side with your hand a couple of times. This will get the air out of the tube and make some water come out forming a vacuum. If you do not do this, the vacuum may not form. It is also harder to form the vacuum if the bottle is only partially filled. Another thing to check is if the washer is in place and does not have any dirt on it or nicks in it.