These black phenolic caps are very nice and useful, but for some reason they can break more easily than we would like them to do so. Please be careful with your bottle so it does not fall over. Tune in to your cap while filling bottle so you can seal your bottle. Happily we have plenty of these caps, but they are extremely challenging to find. They are only made in Europe and there is really only one cap made that fits these bottles!
You could always use a cork if you are waiting for your replacement caps. We like to send 2 caps so you have one on hand for any future losses or breakages, and you can take advantage of just ordering one time to get 2 caps.
Be sure to keep your bottle upright so that your water is not touching the inside of the cap.