This has been discussed before also.
Check the metal plates on your capper. The part that grabs the neck of the bottle. Do they slide out? If they do you'll notice that one end of the cresent cut out is smaller than the other end...or the distance from the end to the cresent is longer.
This means that you have an adjustable clamp set. One for normal sized necks and one for smaller/narrower necks.
Hope this helps.
